Gain of wireless antenna? (-3, 0, 3, 6)
Decibel, or dB, is a measure for a relation of two parameters. \$ Decibel = 10 \cdot log\frac{P}{P_0}\$, with \$P_0\$ beeing the parameter you compare your signal with.

For calculating the gain of an antenna, you compare the recievable power at some point with the recievable power at that same point when a simple dipole antenna is used. When the power in both cases is the same (so there is no gain), than the \$\frac{P}{P_0} = 1\$. The log of 1 is 0. So 0 dB means, the antenna has no gain compared to a dipole antenna. -3 dB means, the antenna is producing half the power of a dipole antenna at the point where you measure. 

To give an absolute measure of power in dB you use dBm. This stands for decibel compared to 1 mW, so you have \$P_0 = 1\ \text{mW}\$. 3 dBm would be double that of 1 mW => 2 mW. And -10 dBm would be 0.1 mW = 100 &mu;W.

Regarding your question about lower and higher frequencies:  

There is a concept called free-space path loss, which is changing with signal frequency. Also the effect you mentioned with 2.4 and 5 GHz WLAN is due to longer wavelengths (or lower frequencies) having a lower attenuation. That's why you receive a 2.4 GHz signal even in the next room, while 5 GHz is blocked quite strongly.  

To calculate the received signal strength with a directional antenna you calculate the overall gain/attenuation of the signal path. This includes the output of the power amplifier, the gain of the sending antenna, the attenuation in air and the gain of the receiving antenna. With all values in dB you can just add them (with gains being positive, attenuations being negative).

\$P_{recieved} = P_{send} + G_{send} + FSPL + G_{recieve} \$