you can assign nearly everything to variables, e.g. a special unit/hero you wanna transport from map to map. For example, you assign your hero to a variable called "MyHero" (the variable type has to be "unit"). Now, other triggers can refer to the unit by refering to it's variable, e.g. a trigger like <<Unit - Kill "MyHero">>
Extensive maps/campaigns can't be done without variables, e.g. transporting a hero from map to map is impossible without variables. ATM I'm writing a tutorial concerning to hero-transport, look it up when it's finished.