What it tells you
Market cap shows how much the market values a network as a whole. A $1 coin is not cheap if billions exist, and a $50,000 coin is not expensive if supply is tiny. Comparing caps is more meaningful than comparing prices.
Where it misleads
Market cap does not mean that much money was invested or could be withdrawn. Thinly traded tokens with most supply locked can show huge caps on small volume. Look at liquidity, volume and fully diluted value too.