what does l in mean for motherboard IO

On motherboard audio I/O, L-IN means Line In : a blue audio input jack for bringing sound into the PC from devices like a mixer, phone, or another audio source. It is different from L-OUT , which is the usual speaker/headphone output jack.

What it’s for

• Connect an external audio source to the computer.
• Record or pass audio into software.
• Not meant for headphones or speakers.

Easy way to remember

• IN = sound goes into the PC.
• OUT = sound comes out of the PC.

If you want, I can also decode the other rear audio labels like CS-Out , RS-Out , and SS-Out.