Does a forty percent chance of rain mean that it will rain?

No.

It's a description of a probability, or a chance, or a risk.

It says it might rain.

100% that's a certainty, like the sun is going to go up tomorrow.

0%, that's something that won't happen, like Elvis returning, or world peace

50%, that's something that's as likely to happen as it's unlikely. Flipping a coin and calling it right is 50%.

40% means that it's a little more likely to remain dry.