My apologies for being remiss - for about a week I have been a non-poster! Gasp! Aieee! Other interjections of horror!
But now for the meat: this is a post explaining what I mean, and what all animators mean, when they talk about animating on 1s, 2s, and 3s (or 4s, or 5s - you could essentially put any number here). First, you must understand that film is composed of a series of photographs, traditionally running through a camera at the rate of 24 frames (or photographs) per second. For any film student, this is a basic fact, like knowing that the first three letters of the English alphabet are A, B, and C. But this is not common knowledge unless you've made some dabbling jaunts into exploring filmmaking or film in general. So, 24 frames per second (or 24 fps) is the rate to remember when you're discussing animating on ones, twos, and threes.
As a side note, for television and digital cameras, there are actually 30 frames per second, which is why the mediums tend to look so instantly distinct from each other - well, one reason of many.
Nevertheless, whether shooting for TV or on film, animation works on the 24 fps timing principle. Now comes the 1s, 2s, and 3s part: these numbers refer to the number of times you photograph each drawing you create (or pose you hold, if you're animating with stop motion) before changing to the next pose or drawing.
That was a confusing definition, so let's make it simpler. If you animate on ones, you draw 24 completely different drawings per second of film - a lot of work, and yet the animation flows very nicely. Disney animated feature films often are animated on ones. If you animate on twos, you draw 12 completely different drawings per second of film, photographing each individual drawing twice so that you end up, once again, with 24 frames per each second of film. Contemporary American animated cartoons on TV tend to animate on 2s because the quality of the animation doesn't suffer too badly, and this method saves money because you're not demanding as many drawings per episode. Therefore, when you animate on threes, you create 8 completely different drawings per second of film, photographing each individual drawing...you guessed it, three times. If twos are cheaper, why not threes? - you might ask. Well, when you start to animate on larger numbers, the animation starts to look jerkier and stiffer, which American audiences, at the very least, won't tune into with any degree of regularity.
So there you have it. Now you know - aren't you a clever bird? Now try and animate something - it's fuuuuuuuuuuun!