I have been reading about lens extenders. If I use a 1.4x lens extender I read that my lens need to be stopped down a stop.
The extender lets less light through to the sensor than the lens on its own, a stop less light with a 1.4x, but in practice you can ignore this if you are using the built in camera exposure meter.
However, the way you have phrased your questions "needs to be stoped down a stop" possibly suggests something else is being referred to.
Most extenders reduce optical quality, and its common to hear people say you need to stop the lens down a stop when using an extender, to try and recover some of this lost quality.
So with a 300mm f4, set wide open at f4 with a 1.4 extender, the light reaching the sensor will be a stop less than it would have been without the extender.
If you get a loss of quality at f4 with the 1.4 extender you may want to stop the lens down to f5.6 to recover the quality and choose to lose another stop of light in the process.
In both cases the camera meter will still be reading the amount of light that actually reaches the sensor, so you don't need to manually intervene.
Whether you "need" to stop the lens down, and by how much, will depend on your personal experiences with the lens+extender, and how you want to compromise between optical quality and using a higher shutter speed that might be needed to stop subject movement or camera shake.
