stands for many words:
important, impossible, impressive, improbable, impartial, impatient, impolite...etc.
Basically you say "that's imp" when someone tells you something and you don't know how to respond. This lets the other person decide what he wants it to mean.
Most often is used to mean impressive.
"I climbed Mt. Everest today"
(did he/she mean important? impossible? impressive? improbable?)