Ok, let's start off with a definition, that I lifted from this site:

Political ecology is the study of the relationships between political, economic and social factors with environmental issues and changes. Political ecology differs from apolitical ecological studies by politicizing environmental issues and phenomena.

So, how does that relate to the colonial era of Africa?

Political ecology is a way of looking at, say, the establishment of Virunga National Park in colonial Belgian Congo. You can look at what political and economic forces went into the creation of the park, like the international conservation movement in the 1920s. Or the interests of European hunters/safari practitioners to have an area where game species could reproduce. Why might hunters push for that protected area? To ensure that there will continue to be game species in the future, and Europeans could continue to go on safari in future. Particularly in light of the drastic decline in elephant populations throughout the 19th century due to the ivory trade.

Extending the political ecology exploration further, we can then try and trace the African populations that lived on the land that became Virunga. Once they were displaced, was any provision made to transition them into new employment/subsistence? Or, did those displaced populations instead turn to poaching due to economic displacement and poverty. Did European administrators understand the economic links of poverty to poaching, or did they treat it as simply a banditry problem?

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Shifting topics slightly, we can also look at agricultural policy. In the colonial era, French and British administrations encouraged the growing of cash-crops, as a way of generating revenue and making colonies self-funding.

In French Soudan (Mali) this meant a large-scale cotton-growing scheme, which was facilitated by widespread use of corvee labor to dig the irrigation canals.

Thus, a French developmental scheme (i.e. a political undertaking) meant to generate export revenue on cotton (an economic dimension), had profound impacts on the distribution of water resources in the middle niger river, and encouraged mono-crop cotton production at the expense of more diversified subsistence agriculture of the pre-colonial era.

A similar story can be told about Anglo-Egyptian Sudan, with British developmental policy encouraging extensive irrigation schemes of the upper Nile, and encouraging food production to support growing populations in neighboring Egypt. In the post-colonial era, successive Sudanese regimes followed this policy, and to an extent came to equate the construction of dams and waterways as means to the development/advancement of the nation. Like with Mali, this had profound impacts on the land and water resources, and promoted a type of intensive, export-driven agriculture that led to soil depletion. Harry Verhoeven discusses this extensively in Water, Civilization and Power in Sudan.

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A more recent example might be the development of Bauxite mines in Guinea or conflicts over oil in the Niger delta.

Political Ecological analyses of these stories would examine the national motivations for allowing mining. Motivations like tax revenue, jobs, development.

We could also examine these projects impacts on the surrounding communities and environment. Health damage due to bauxite dust, oil spills, as well as violence because of oil-pilfering gangs and rebel groups who are aggrieved because local communities do not see financial benefits of oil production.

A political ecological analysis would ask: what groups benefit politically or financially from the environmental degradation from mining. What groups suffer from this environmental degradation, and why is there this differential?

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So yeah, at its heart, political ecology is an examination of how power dynamics impact decisions of development vs ecological conservation. It questions who benefits and who suffers from policies.

There can also be unintended consequences. For example, the colonial period resulted in the spread of trade connections deep into the continent, so Africa was connected to the wider world to a greater extent than before. An unintended result of this greater connection was that there were frequent spreads of epidemics, both affecting humans like Cholera epidemics (see Africa in the time of Cholera for more). As well as spread of epidemics like Rinderpest that decimated livestock herds. With the death of large numbers of humans and animals, areas that had been farmed fell out of cultivation, and areas that had been pasture were changed when livestock died. So, unintended and unexpected impact on the environment proceeding from a policy of expanding trade and colonial domination into Africa.

Hope this explanation makes sense and helps!