second year wood. A shoot that grows this year will get blossoms and fruit next year under good conditions (no late frost, etc). Pruning substantially in the winter stimulates these new shoots to grow. Pruning in the summer will limit growth so controlling the size of the tree, etc can be accomplished by summer shaping and pruning. Prunes need light so keeping center of tree somewhat open is good. Pruning out most shoots that grow toward center of tree should be good. Many stone fruits produce way too many fruits that end up being small and may drop early. Thinning the fruit so each has some space is usually a good plan. When they are about marble size thin voraciously. Also if they aren't pollinated properly they may drop. Some trees are "self fruitful" but most require another tree to pollinate correctly. Do you have another plum tree?? Might need to get one!
Lots of you tube videos about pruning peaches and plums. Google is your friend. Good luck.