Basically and in a nutshell. A flat rated engine is one that is restricted to a maximum thrust even though it is capable of producing more due to lower ambient temperatures than ISA, higher density, etc. Therefore, at temperatures below the temperature at which the engine is flat rated to, the thrust is not affected by temperature. A flat rating is normally ISA +15 degrees celcius.
In other words, from low temperature, all the way up to 30 degrees celcius you get the same amount of thrust.
Hope that helps.