Leaf Cells could be standard cells from an ASIC. library or memory or special macro cells. These are the base cells that are used for further design/layout. Like you design the leaf cell first and then use multiple instances of it to create larger blocks.
Leaf Cell is the cell in lowest level. i.e. it doesn't contain any hierarchy. but we can use some of it to build our design. just like the tree, the branch has a lot of leafs