every post you make that is seen by at least one other instance other than yours, it saved as a copy on that other instance.
That’s basically how instances communicate with each other, If you as a lemmy.world user go to lemmy.ml, lemmy.world get’s a copy of the content of lemmy.ml. In the same way if you post something direclty on lemmy.world, that post will be included in copys of every other instance viewed lemmy.world after you made that comment.