Six is the count that trips people up on rows versus columns, so here is the convention this tool uses everywhere: rows first, then columns. 2 by 3 means two rows and three columns. 3 by 2 means three rows and two columns. They give six tiles either way, but the tiles are shaped differently and they go in different places.
2 by 3 suits a wide source. A 3000 × 2000 photo becomes six 1000 × 1000 px tiles, each keeping the 3:2 shape of the original. On an Instagram profile, that is two full rows of the grid, which is the standard way people build a banner across the top of a feed.
3 by 2 suits a tall source: a portrait photo, a poster, a long menu. Six 1000 × 1000 px tiles from a 2000 × 3000 file.
1 by 6 is the six-slide carousel or the six-panel strip. Six panels, one row, posted left to right.
Dragging changes the maths but not the count. Move any of the interior lines and the tiles either side resize. That matters more at six than at four, because six tiles means more chances for a cut to land on someone's face.
All splitting happens on a canvas inside this page, so your image never leaves your device.