A basic question about the meaning of dB of an antenna radiation pattern
See the point at the end of the lobe - it's called the "peak output power point". The point called "-3 dB from peak" tells me that at this point, if there was an RF power density of (say) 100 watts per sq metre at the peak point then there will only be 50 watts per sq metre at the -3 dB point.

-3 dB when converted to a power ratio is \$10^{-3/10}\$ = 0.50119 (or basically a half if pin-point accuracy isn't required).