what does shallow mean?

Yes.

Don't dive into shallow water. It is dangerous.
Shallow water freezes more quickly than deep water.

His thinking is shallow, not deep.

is it positive or negative adj?

thanks again!

AliaAhmadis it positive or negative adj?

I'm not sure what you mean by positive or negative.
Shallow is opposite of deep.

I cut myself with a knife accidentally.
a) The cut is shallow, and will heal quickly.
b) The cut is deep and needs stitches, so I have to go to the doctor.

When used figuratively, for example of a person, it tends to be disparaging.

ozzourtiWhen used figuratively, for example of a person, it tends to be disparaging.

That's true. A shallow person would be frivolous, not thinking about the consequences of his actions.

AlpheccaStars A shallow person would be frivolous, not thinking about the consequences of his actions.

That's right, and 'someone who is deep has very serious thoughts, ideas, or feelings'. (Macmillan)

Rover_KEThat's right, and 'someone who is deep has very serious thoughts, ideas, or feelings'. (Macmillan)

I've never used it that way - ie John is a deep man.
But I have used "deep in thought" quite a bit.