The big left part in the schematic of the jack is its barrel. It is the longest part of the plug called the sleeve. It is 0V.
The L terminal (the tip of the plug) is the left stereo channel and the R terminal (the ring of the plug) is the right stereo channel.
A 3.5mm stereo jack can be an input on a computer from an MP3 player, an output to headphones or an output to little speakers.