old D&D rules "multiclassing" were extremely restrictive, complex and error-prone. you couldn't just decide to take your next level as "whatever" as you could in 3.X
Up to 2nd ed, you could be a multi-class demihuman, but you had to choose the classes at character creation from a preset number of racially restricted choices. XP would be split between your 2-3 classes, and since each class had it's own unique level up table, the classses would level up at different rates.
Or, you could start as a single class human, then at some point, switch to another class but you're bumped back to first level and not allowed to use any of the old class's abilities until you get your new class back up to the level of the old class. This was how the bard thing worked in 1ed.
e.g. you could start as a fighter, then switch to a mage at level 4, but you had to somehow forget how to hold a sword and wear armor, then when you get mage back to level 4 and above, you were suddenly allowed to use your fighter abilities again. Which was a bullshit system, but obviously could be an advantage over playing a pure mage: start as a class with strong HP for a few levels then switch to a magic-heavy class later. Since early levels are so cheap you only end up about 1 level behind everyone else, but you can gain quite a few levels of fighter for that one lost level of mage (because each class had it's XP calculated separately).
Obviously, the 3ed+ system is a lot simpler than the above choices.