What's the definition of tonicity

What's the definition of tonicity?
I want a definition that's not so confusing with a bunch of big words, lol. Like, not a dictionary definition, but a definition used to explain to like a 10 year old. Thank you!

Tonicity is a measure of the amount of substance dissolved in a liquid (in most cases water). Take an example of salt. The more salt is dissolved in water, the more it is HYPERTONIC (a thick dense solution). And the lesser salt is dissolved, it is HYPOTONIC (a thin dilute solution). Hope this is not complicated.