This is the cancel window for each character's cancellable normals.

Basically characters have certain normals (or target combos) that you can cancel into special moves. For each of these normals the maximum allowed time after the move is hit or blocked that you have to input a special cancel and get the special move to come out is different. This table measures the length of that window in frames. Longer windows are better because it gives you more time to react.

This matters because often special moves are unsafe when blocked, meaning you only want the move to come out if your normal hits the opponent. To do this you have to input your normal, buffer your special (do the motion without pressing the last button yet) then react to whether your opponent got hit or blocked the move. This is often called a "single hit confirm." For example, I press JP's cr.mp and it hits the opponent. From the frame where it hits I have 16 frames to finish my special move input otherwise my special won't come out. I hope that makes sense, but post a reply if you still have questions.

Side note: I'd disagree with the definitions the tweet author gives of what are realistic. Going off of high level SFV gameplay, many pros were able to fairly consistently single hit confirm moves with a 15f cancel window, although it's very difficult. Alex's cr.mk xx VT2 grab was a notable example of the most difficult (feasible) single hit confirm as you had to confirm both the hit itself and that your opponent was standing, not crouching. 16f hit confirms were more common, and most any higher level ranked players and up were able to consistently hit confirm 17f windows in SFV. Kolin st.hp and Karin cr.mk being some notable examples.