Signifier and Signified
Lacan uses the verb "slip" -- and its corresponding noun, "slippage" -- to describe the unstable relationship between the signifier and the signified.
The term thus emphasizes the different ways in which Saussure and Lacan conceive of signification; for Saussure, signification was a stable bond between signifier and signified, but for Lacan it is an unstable, fluid relationship.
It is impossible to establish a stable one-to-one link between signifiers and signifieds, and Lacan symbolizes this by inscribing a bar between them in the Saussurean algorithm.
Point de capiton
The signified slips and slides under the bar of the Saussurean algorithm in a continuous movement, a movement which is only temporarily detained by the points de capiton.
When there are not enough points de capiton, as is in the case in psychosis, the slippery movement of signification is endless, and stable meanings dissolve altogether.