Energy breakeven occurs when the energy released by the fusion reaction equals the energy put in to heat the plasma (i.e. for a D-T plasma: alpha particle power plus neutron power).
The fusion performance of a power plant is denoted by Q, which is the ratio of the energy of the fusion products to that used to heat the plasma. While breakeven corresponds to Q = 1, a burning plasma has Q > 1 and ignition corresponds to Q = infinity.
Read more about breakeven on FAQ: How much more energy will it take a Tokamak over the “breakeven point” to actually produce power?