MWe and MWt are units for measuring the output of a power plant. MWe means megawatts of electrical output, and MWt means megawatts of thermal output.
For example, a nuclear power plant might use a fission reactor to generate heat (thermal output) which creates steam to drive a turbine to generate electricity (electrical output). See nuclear proliferation for discussion of a reactor that generates 200 MWt (50 MWe), and another reactor that generates 800 MWt (200 MWe).