Shortly after the PC became popular with enthusiasts, sometime in the early to mid-1980s if I recall correctly, there came a plethora of "sound cards" such as Sound Blaster featuring "line in" "line out" and "microphone" jacks. More modern PCs have these same jacks built into the motherboard as part of an integrated sound system. Depending on the application software, the jacks may be programmed to serve different purposes, such as (for a 6-channel audio) a bass or center speaker channel instead of microphone in, front channel speakers instead of line out, and rear channel speakers instead of line in. However, assuming the alternate definitions are not active:

The "line in" jack accepts a high-level audio signal, typically generated from a tape deck or other analog audio equipment, and presents it for digitization by the sound card or integrated sound system on the PC.

The "line out" jack provides a high-level analog audio signal, provided by the sound card or integrated sound system on the PC, when you play digital content with a media player. That would be something like an MP3 file or any other digital audio file for which your PC as a CODEC installed. This signal is appropriate for connection to an amplified stereo speaker system.

The "microphone" jack is a low-level analog audio input typically derived from an external microphone. It functions identically to the "line in" signal input except it accepts and digitizes a lower signal level. In other words it is more sensitive than the "line in" signal input.

Sometimes the jacks are color coded: pink for microphone, light blue for line in, and lime for line out on my ASUS P4B266 motherboard with integrated sound system. The line out jack often has a loudspeaker symbol next to it. Best way to find out which one is "line out" is to connect an external amplified speaker system and try each jack until you find the one that plays content from your PC media player. Use appropriately low volume-control levels while doing this.

Some internal audio CD drives provide a stereo analog output that connects to the sound card with a little three-wire cable. This allows audio CDs to play directly through the external speakers connected to the line out jack. It may also facilitate ripping music from audio CDs. The comment has nothing to do with the OP's question. I just threw it in there for "completeness."

By suitable of adjustment of the signal levels (think 10 kilo-ohm potentiometer) you can use the "microphone input" jack the same as the "line in" jack. The analog-to-digital converter on the sound board, or on an integrated digital sound system, has a maximum input signal level that will in turn result in the maximum digitized "word" the PC is capable of storing and manipulating. Higher input levels produce exactly the same maximum digitized word and therefore the audio information content is lost. An audiophile purist will call this "clipping" and it is undesirable anywhere except for heavy metal stage performances.