"Plum pudding" was the descriptive name of the atomic model developed by physicist J.J. Thomson at the beginning of the 20th century. Thomson, who you might have guessed was British, discovered the existence of electrons (he called them corpuscles
) in the atom in 1897. He and his fellow scientists knew that electrons were negatively charged, but they couldn't immediately explain why, then, most atoms carried a neutral charge.
They hypothesized that atoms must also carry some sort of subatomic material that was positively charged, to balance it out. (Bear in mind that physicists had not yet discovered that atoms contained a nucleus.) Thomson ran some tests and, in 1904, proposed what came to be called the plum pudding model of the atom. In this model, the atom was made up of negatively charged electrons (the plums) surrounded by a sphere of positively charged fluid (the pudding).
Thomson's model was disproved five years later, but his model and experiments were important stepping stones in atomic research. For his efforts, J.J. Thomson was awarded the 1906 Nobel Prize in Physics.