What did that mean

No he did not see it. They were showing the flashback. As in how that 4koma had entered his room when they were in elementary school. And he picked it up, read it, recognised it and then ran after her

When fujino ripped her comic and slipped down to kyomoto's bedroom door it's implied (literally shown) that the piece of paper comic saying "don't come out" was sent to an alternate universe of theirs, and because of that paper kyomoto didn't come out of her room. Fujino believes that because of her four-panel comic, kyomoto got out of her room, creating a butterfly effect which led kyomoto to her murder-death, (from what I believed) either fujino imagined what will happen if kyomoto didn't leave her room and if she continued karate, she would have stopped the death of kyomoto because she continued karate and because the karate school was next to the art school OR the paper piece was actually sent to an alternate universe. After that we see kyomoto making a small four panel comic like fujino, making a comic of their first time meeting together at the art school and fujino saving her. (And from what I believed) Our kyomoto in the real world created that four panel comic when she was a kid basically and pasted it on her window (the drawing of that panel was like a kid-like drawing) ironically that four panel comic of hers kinda happened but kyomoto isn't saved by fujino and she dies. That four-panel comic by kyomoto, unsticks from the window and slipping out of her bedroom door, fujino sees and reads it. She reminiscents the times she had with kyomoto then goes back to her apartment and draws for the next volume of her manga.

Thanks! I wasn't sure if it was time travel or parallel universe and got confused

The first time I read it I kinda got confused as well I thought the manga was set in a normal world so I got surprised that there was an alternate universe. But with further analysis. I think the drawing fujino saw was really just an old drawing made by kyomoto when she was a kid.