The flex is the flex temperature, basically a measure of the derate on the take off thrust. You tell the aircraft how hot you want it to be outside and the engine provides what it thinks is the maximum thrust it can give you if its that hot, hence you get less than the maximum available thrust. You should get an indication of the target flex N1 on the engine display during the take off roll and you can compare this against the actual N1 on the gauges to determine whether the engines are providing the correct power.