It refers to the way the game is built. Basically, every 21 frames the game will check if the player has finished the level. If the player has finished the current level, the next level will load right away, no problem. If the player hasn't reached the end within that set of 21 frames, then there is a waiting period until the next check occurs, which then adds time to the counter. The 21 frame loading rule can also act as a buffer: even if a player technically plays slower than another runner, so long as the level is beat within that same frame window, they'll achieve the same final time for that level. Darbian has an analogy that perhaps will make it easier to understand:
Imagine that the castle Mario walks into at the end of each stage is a bus station, and that Mario must take a bus to the next stage. In the Mushroom Kingdom, buses operate on an oddly specific schedule, such that every 0.35 seconds a bus departs. When Mario reaches the end of a stage, he must wait for the next bus to depart.
When it comes to speedrunning this game, the frame rule means that you don't have to play each level perfectly to get a perfect time, you just need to play each level fast enough to get on the same bus as someone who did play it perfectly.
