Writing questions and answers

Show don't tell is when you don't describe a character trait but let the reader discover the trait by seeing the character an action.

EXAMPLE:
Don't say:
John was very anxious
But:
John paced through the room then he sat down again. After looking at his feet for a couple of seconds he stood up and started pacing again.

Don't say:
John is smart
But:
John could see the solution immediately, it was simple really just a matter of approaching the problem with a fresh perspective.

These make your character a lot more believable AND makes it more real for the reader. Things like "smart, anxious, strong, ..." those are all relative. Just because you, as a writer, consider smart does not mean your reader will.

I hope this helped!