This page describes how the application blocks work.

Each application installation takes up a set number of "blocks" on the device. Individual blocks are shown grouped so it's easier to count them. They will appear on the Item Details screens. Here are some groupings you might commonly see: You are allowed a maximum of 125 application blocks on your device. If you try to purchase an item when you've already reached or are about to exceed your quota, then you will receive a message that you must uninstall some blocks before you can purchase another item. You will be told how many blocks you'll need to uninstall.