AM is ante meridian and PM is post meridian. It’s referenced to the prime meridian through Greenwich UK. If the sun is to the east of the line (ie. Before noon where it is vertical) it’s before (ante) the meridian. If it’s after (post) noon it’s after the meridian. It’s all outdated but that’s how time works.
I just put mint on a 2015 dell shit laptop that barley functioned with windows. Now it’s a perfectly fine computer. I don’t do much besides use the internet but it struggled with that before.