Some computer science terms I don't understand (I'm not naive English speaker) plz help me

''Conditional allows us to write programs that are more interesting than straight- line programs, but the **class of branching programs (1) ** is still quite limited. One way to think about the power of a class of programs is in terms of how long they can take to run (2)"
I dont really understand the last sentence.
intrinsic difficulty of problems (3)
programs of arbitrary complexity (4)