Nine tiles is the Instagram grid split. One picture spread across nine posts, filling three full rows of a profile, is still the most-used trick for turning a feed into a billboard.
Because the grid is square, this preset crops to 1:1 before it cuts. A 1080 × 1080 px source gives nine 360 × 360 px tiles, which is the size Instagram wants and the size it stops rescaling at. Feed a rectangular photo in and you choose where the square crop sits before the nine cuts happen.
Posting order is the part people get wrong. A profile grid fills from the top left with the newest post, so you upload in reverse: bottom right tile first, top left tile last. The tool numbers the files in that order so you can work down the list without thinking about it.
One warning worth knowing before you spend an afternoon on this. Instagram now crops profile thumbnails to 3:4 portrait rather than square, so a nine-square grid shows with the top and bottom of each tile trimmed in the grid view. Tapping a post still shows the full square. Keep faces and text in the middle band of each tile and the grid survives the crop.
Outside Instagram, nine is the photo wall count: three by three frames on a stairwell, or nine sheets taped into a poster.