Erratics are boulders which have been transported by glaciers to locations of a different geology. Some erratics are found hundreds of miles from their source area. They are useful indicators of ice-flow direction.
There is no need to know a named example of an erratic for GCSE Geography, but if you must know, the best-known examples in England are the Norber erratics in Yorkshire. There are also many examples in the Cairngorms.