A placement is a play (tesuji) inside the opponent's group without touching other stones. Its aims are many: killing, yose, sacrifice to get sente etc. Of course, it may fail as a speculative invasion.
Black has approached White's corner with . If White does not respond, Black can play the placement of , after which Black can connect out, either at a or at b, turning White either into a drifting group or small life in the corner.
Here's an example of a placement in a life and death situation:
White is dead due to bent four in the corner.
makes the second eye false and kills the entire group.