The m command is a tricky one to break down. The basics of how it works is that you use it to mark a point, or a section in a text file.
Once you decide where you want to place a marker, you can type m followed by any letter in the alphabet. In other words, there are 26 available markers that you can use.
To navigate to a marker you use the backtick ` followed by the letter marker you used.
The single quote followed by the letter marker you used will take you to the beginning of the line where the marker is.
If you want to set a mark that will jump back to a file you've opened in Vim, you can travel between files with a marker. Say in one file you have a marker with the letter a. In and a different file you can associate a spot for that marker by typing m A, (I believe you have to capitalize the letter for the marker to work across files).
This will show you a list of marks. It will show the ones you have created plus the ones Vim already has as defaults like the numbered ones.
If you want to delete the marks you can type :delm followed by the mark you want to delete.
If you want to delete all marks and reset the marks back to their defaults, you can type :delm!
The angle brackets are a special kind of marker. The let let mark a block. The left angle bracket will mark the start and the right angle bracket will mark the end. One bracket by itself will just take to that spot, but if you do g follow by v, it will select that blocked that you marked.