the if statement is a conditional , which means those conditions must be met before it will continue to execute anything under it.
eg:
if (is_user_alive(id) && cs_get_user_team(id) == TERRORIST)
{
>>>execute the code that is here
}
is saying that those 2 conditions must be met before if will execute the code inside { }
things like || , && , == , >= mean
and, or , equal to , greater then or equal to
you can learn about them here
https://wiki.alliedmods.net/Pawn_Tutorial#Conditionals