Grace jumping also known commonly as coyote jumping is when you go of the platform and jump even when not touching the ground which is normally intended by the developer. As for 1300m, it depends whether you consider doing a grace jump after a dash allowed or not. If you don't, you would do it as intended by jumping up to the first balloon, dashing diagonally and so on since there's no solution to avoiding jumping in 1300m. If you consider it to be allowed, follow the video posted by Parchment. If you still don't understand, you can search up the celeste classic glossary which might explain it better.