Here i found
I was wondering for what it stand for is it for video quality ?
preset is a private option that may be defined for an encoder, such as libx264. It is a shorthand method to set a whole bunch of options at once, and won't have any effect if set when using an encoder which doesn't define them.
When using x264 to encode to H.264 with preset medium, these are the arguments set:
cabac=1 ref=3 deblock=1:0:0 analyse=0x3:0x113 me=hex subme=7 psy=1 psy_rd=1.00:0.00 mixed_ref=1 me_range=16 chroma_me=1 trellis=1 8x8dct=1 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=-2 threads=12 lookahead_threads=2 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframes=3 b_pyramid=2 b_adapt=1 b_bias=0 direct=1 weightb=1 open_gop=0 weightp=2 keyint=250 keyint_min=25 scenecut=40 intra_refresh=0 rc_lookahead=40 rc=crf mbtree=1 crf=23.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=1:1.00
The default preset is
medium. You can override any of the individual variables by adding
-x264opts key=value after the preset option. But you shouldn't unless you need to.
On the page you linked to it explains it fairly well:
A preset is a collection of options that will provide a certain encoding speed to compression ratio. 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. Similarly, for constant quality encoding, you will simply save bitrate by choosing a slower preset.
Basically there are three variables: speed, quality and file size that all depend on each other. With constant quality encodes (using the
-crf option) you can change one of the other two. With the same quality setting if you increase the speed then you increase the file size, and if you decrease the speed you will get a smaller file.
medium preset is is a balance between encoding speed and file size. You can tip the balance either way, if you have time you can change the preset to slow, slower, veryslow or—for a tiny improvement in file size with big increase in time—placebo. Or if you need something converted on the fly you could increase the speed all the way up to realtime.
But all of the encodes will have the same quality. You change your quality only with the
-crf setting. Of course lowering the quality will result in smaller files and vice versa, but that has nothing to do with the preset setting.