To know what I mean just look at a map of the Roman Empire.
Take a look at a topographical map of Africa.
http://en-gb.topographic-map.com/places/Africa-353128/
The Romans essentially occupied the areas up to the mountains and tried to control the flow of tribes into and out of the more settled (and extremely rich) areas, beyond that the landscape was to marginal and poor to be worth going much further or support development with a more nomadic and dispersed population.
There are more extensive answers here from /u/cleopatra_philopater and /u/Iguana_on_a_stick https://old.reddit.com/r/AskHistorians/comments/7q8l57/to_what_extent_were_the_romans_in_contact_with/
https://old.reddit.com/r/AskHistorians/comments/8lw411/how_and_why_did_rome_hold_north_africa/
With this question specifically also answered by /u/Claystead here