Peasants, mostly. Civilians. The basic idea behind General Valeriano Weyler's policy of forcing virtually everyone in the countryside to move into the cities was that peasants fed, housed, and supported guerrillas in other ways. No peasants, no problems.

If someone from a region which had been ordered to move into the cities was found in the countryside after a certain date, they'd be treated as an enemy combatant.

What made the policy infamous was that the Spanish moved the civilian population of the Cuban countryside into cities without actually providing for the people they had turned into refugees.

No money to buy goods, no food to eat, little to no housing support. Just 'move to your local city and stay there until we solve this rebel problem'. Predictably, many thousands starved to death. Many others were malnourished and prime targets for diseases which had long plagued the island but were particularly deadly given the poor health of many peasants and how concentrated they were in urban spaces.

I've seen death tolls anywhere from the tens to the hundreds of thousands, but as it isn't my specialization I can't tell you where scholarship currently stands as to the 'true' number of deaths caused by this policy.

A good place to start would be Louis Perez Jr.'s Cuba: Between Reform and Revolution, a manual of Cuban history for undergrads and non-specialists.