The default timing of 2D animation is 24 frames per second. This means that there would be 24 individual drawings for every second’s worth of animation.
Frames, in general animation context, refer to drawings.
That said, professional animators have found ways to streamline processes and save time. These processes are called “Animating on Ones, Twos, or Threes”.
Meaning, frames can be held, so the same drawing would be shown for multiple frames, rather than having to make a new drawing for every single frame.
Check out the definitions of ones, twos, and threes below, and then refer to the GIF at the end to get a better idea of what the differences between might look like.
Animating on 1s
Animating on 1s means that for each second of animation, there are 24 new drawings or “frames”. This is used most often in action sequences or any fast motion that needs to also be detailed.
Animating on 2s
Animating on 2s means that for each second of animation, there are 12 new drawings or “frames”. This is the most common type of animation. This timing of animation is best for doing simple everyday motions. This should be your default way of animating until you become more familiar with timing in animation.
Animating on 3s
Animating on 3s means that for each second of animation, there are 8 new drawings of “frames”. This timing is good for slow scenes but is also often used in anime.
You can mix these differing timing as needed in your animation, you don't have to stick to just one set.