A plum pudding is a dense, boiled or steamed, pudding containing spices and various types of dried grapes (raisins, sultanas, etc.).
Centuries ago, it contained dried plums, and that's where the name comes from.
In the 16th century the plums began to be replaced by dried grapes, but the pudding was still called "plum" pudding.
A plum pudding looks like that in the picture below.
J. J. Thomson's "plum pudding model" of the atom would look something like this: