These are the core Heat of Battle files. You'll need this and at least one map pack in order to play!
To install, extract to the folder that contains your BF2 installation. This folder should contain a folder called Battlefield 2. For a box copy, the folder should be called EA GAMES; for Revive, the folder should be called Revive; for BFHD, the folder should be called BF2HDRemastered or similar.
Make sure to install the mods/bf2 folder as well as the mods/Heat_of_Battle folder. The mods/bf2 folder contains the ESAI files which are required. If you don't have them, you will crash when joining a game.
Please see the installation article for more details: Moddb.com