A fusion energy gain factor, usually expressed with the symbol Q, is the ratio of fusion power produced in a nuclear fusion reactor to the power required to maintain the plasma in steady state. The condition of Q = 1, when the power being released by the fusion reactions is equal to the required heating power, is referred to as breakeven, or in some sources, scientific breakeven.

TL;DR: high Q = free unlimited energy!!!! low Q = "fusion is 30 years away"

https://en.wikipedia.org/wiki/Fusion_energy_gain_factor