Positive dBm means power greater than 1mw and negative means less than 1mw.

Why is dBm always negative?

Signal strengths for mobile networks are always negative dBm values, because the transmitted network is not strong enough to give positive dBm values.

How can you have negative dBm?

Likewise, a negative Decibel-milliwatt (dBm) means that you're applying a negative exponent in your power calculations; 0 dBm equals 1 milliwatt (mW) of power, so -10 dBm equates to 0.1 mW, -20 dBm equates to 0.01 mW, and so forth.