In the timeline and in clips I use markers a lot. Mainly I use them as points to sync things up to and as sticky notes to remind me to do something to the footage at that point.
If you don't have a clip or anything in the timeline highlighted and you tap "M" you'll add a marker to the timeline. Tap "M" again to bring up it's window where you can enter in a note or set it to be a specific kind of marker. This kind of marker will stay put in the timeline.
When you put the playhead over this marker you'll see what you typed in appear in the Canvas.
But, if you have a clip highlighted and you tap "M" you'll add a marker to that clip so that marker will move relative to the clip. Like with a timeline marker tap "M" again to bring up it's window. However, unlike a timeline marker, a clip marker won't show up in the Canvas as far as I can tell.
Two handy keyboard shortcuts are Option + M to move to the previous marker and Shift + M to move to the next marker in the timeline. These work for clips too if you have a clip highlighted.
Oh, to delete a clip put the playhead on it and tap "M" then select Delete in the window that pops up.
If you want to delete ALL of the markers in your timeline tap Control + ' (it's called a grave accent mark on the so-called "tilde" ~ key).
Btw, if you want to export a list of your markers there's a free tool for it that a few people have mentioned here.