what do you mean by depressants?

Depressants: These are the chemical compounds used to check the action of one compound in the mixture of two or more. The depressant simply means a molecule which inhibits or sustain another molecule. 

These depressants are used in Froth Floatation Method of extraction of sulphide ores. The depressant check one of the sulphide ores to make the froth with the solution so that the other one is easily extracted. If it is not added , then both the sulphides will create the froth and the extraction would be difficult.

For ex: In mixture of ZnS and PbS, the depressant NaCN is added which makes a complex with ZnS and it does not make the froth with the solution.

4NaCN +ZnS ---> Na₂[Zn(CN)₄] + Na₂S