Question

what are poisons in chemistry and what are its uses

Answer 1

Catalyst poisons are the one which reduces the activity of the catalyst that means they act as negative catalyst. Sometimes they deactivated the catalyst fully or partially.
Ex., BaSO₄ acts as catalyst poison for palladium in Rosenmund's reaction. Other wise the aldehydes are further reduced to alcohol
$RCOCl+H_2\underset{BaS{O}_4}{\overset{Pd}{\to }}RCHO$
Another catalyst is Lindlar catalyst.