Frequency separation technique helps you to retouch a portrait easier in Photoshop.
Frequency separation allows you to separate the texture of an image from the tone and color. When you are retouching you can concentrate just on the texture or on the skin beneath it.
It seems complex but in practice it is really simple. If you want to work with texture you have to choose the high frequency layer, if you want to work with skin just choose the low frequency layer.
In this tutorial of PHLEARN.COM you can learn the FS technique in a very easy way.