Polarisation, in general, just means that one side has become positive, and one side has become negative (i.e.: a "positive" pole and a "negative" pole has formed, hence polarisation)

In the context of primary cells, it occurs when a build up of charge occurs, which then inhibits the flow of electrons: negatively charged electrons will flow to one end, causing a build up of negative charge, and hence polarisation, but this then repels other electrons from coming, so the battery stops producing energy!

To prevent this, a salt bridge can help, as this helps to balance the build up of charges, so as to prevent polarisation from occuring.

The dry cell is an example of where polarisation happens.