The first paragraph on the antenna theory page has multiple errors. Interestingly though the paragraph right below figure 1 does get the relationship right.
In the above Figure, S21 represents the power received at antenna 2 relative to the power input to antenna 1. For instance, S21=0 dB implies that all the power delivered to antenna 1 ends up at the terminals of antenna 2 (which isn't physically possible). If S21=-10 dB, this implies that 1 Watt delivered to antenna 1 (or 0 dB), ends up as -10 dB at antenna 2, or 0.1 Watts.