1.google it.
2.hex editing is using a program called a hex editor (alot like notpad) to alter code in games and make stuff happen that isnt supposed to by default.
3. Purpose is some games dont have tools to mod them, in the beginning of swbf 1 all mods were done using kex editors and notepad.
4. yes , it is. there are several free ones on the internet.
Most of the mods made with hex editors just move things around, add or remove units and gave new skins but nothing super amazing. although it was cool with no tools out at the time , to have all kinds of insane mods popping up!
Once tools came out for this game i stopped modding it.

old school is fun.