A dwarf fruit tree is a shorter version of a fruit tree. This can either be done through genetically making it smaller.Or a naturally pruning the tree to grow up to a certain height in which it will still produce abundant amount of fruits.
There are citrus dwarf fruit trees in which you can buy, that you will be able to grow inside. But since you live in California you can pretty much grow it outside all year. Lemon Dwarf trees and other citrus dwarf trees are pretty easy to grow.