This article is from the Go FAQ, by Morten PAHLE firstname.lastname@example.org with numerous contributions by others.
Shape is a concept often mentioned in Go.
Recognising 'good' or 'bad' shape when you see it, and being able to
play good shape moves will make a big difference to your game.
Unfortunately, shape is not a subject which is easy to understand, and
it is also not something which you can 'prove'. E.g. the fact that a
group is dead can be proved by playing it out. However, a good shape
is only good because it is good. It may never be able to show how good
a shape it is, yet it will still influence the goban around it. So
goodness of shape is an intuitive concept, a matter of experience. If
a stronger player tells you that some shape is good, believe him. You
will get a feeling for what he meant as your Go strength increases.
(If you ever understand 'shape' fully, please let me know! You could
make a fortune writing a book on it :-)
Some explanations are here: