"Impressive" describes an accomplishment that you think is good, which not many people are able to do. You say this to praise someone in a slightly formal way.
A: He built the whole thing himself.
B: Oh, wow. That's so impressive!
It's common to add words like "so", "truly" or "really" before "impressive":
That is truly impressive. I'm so proud of you.
You can also talk about "an impressive (something)":