That's a science unto itself.
The number typically denotes engine displacement such that 300 is 3 liters and 560 is 5.6 liters but there are obvious exceptions such as the 2.6 liter 300E 2.6 and less obvious exceptions such as the early 70s 250 with a 2.8l engine and the current S600 with a 5.4l V12. The AMG series cars also denote the displacement although with only 2 digits (E55, C43, etc.)
TYPICALLY... S in a sedan means sonderklasse or S-class, S in a two-door car means sport. E used to mean einspritzung or fuel injection but later denoted the executiv or E-class. L in a sedan means lang or long, L in a two-door car means licht or light. C means coupe. K means kompressor or supercharger.
I'm recalling the German words from a rusty memory so check twice before you put a bet on anything I say
... D is for Diesel and that's all the MB jargon anyone needs to know