**What?** the act of converting one of the streams (audio or video) from one CODEC to another one.

**Why?** sometimes some devices (TVs, smart phones, console and etc) doesn't support X but Y and newer CODECs provide better compression rate.

**What?** the act of converting from one format (container) to another one.

**Why?** sometimes some devices (TVs, smart phones, console and etc) doesn't support X but Y and sometimes newer containers provide modern required features.

$ ffmpeg \

-i bunny_1080p_60fps.mp4 \

-c copy \ # just saying to ffmpeg to skip encoding

**What?** the act of changing the bit rate, or producing other renditions.

**Why?** people will try to watch your video in a `2G` (edge) connection using a less powerful smart phone or in a `fiber` Internet connection on their 4K TVs therefore you should offer more than on rendition of the same video with different bit rate.

**What?** the act of converting from one resolution to another one, as said before transsizing is often used with transrating.

**Why?** reasons are about the same as for the transrating.