The rate control is a method that will decide how many bits will be used for each frame.
This will determine the file size and also how quality is distributed.
Each frame gets the bitrate it needs to keep the requested quality level.
A slower preset will provide better compression (compression is quality per filesize).
This means that, for example, if you target a certain file size or constant bit rate, you will achieve better quality with a slower preset.
A lower value is a higher quality and a subjectively sane range is 18-28.
Consider 18 to be visually lossless or nearly so: it should look the same or nearly the same as the input but it isn't technically lossless.
Most non-FFmpeg based players will not be able to decode lossless (but You Tube can), so if compatibility is an issue you should not use lossless.