MSL, or mean sea level, is the altitude above sea level. Since AGL, or above ground level, changes as you go over differing terrain, we use MSL to set our altimeter so we are on a common measurement. For instance, where I live, in the Willamette Valley of Oregon, our elevation is only about 200' above sea level. If I am at 1200' msl in the pattern at my airport, I am 1000' agl. Think of agl as msl minus the current terrain elevation. HTH
AGL has to be computed. If you look ar your airport info (like the flight guide) it tells you the airport elevation (mine is 210'). We usually don't know the elevation everywhere we are flying over, so we use MSl unless we have restriction that say we have to stay, say 2200' above someplace (like a wildlife refuge) then you can gusss that the airport is 200' then I better stay 2200' msl in that area.