But why does this happen if it's not a star?
Every tile has an ID. The first tile in the B tileset has id 0. The first tile in the C tileset has id 256. Then it goes D, E, then A5, then the A1 to A4 tiles.
Your map is made up of layers. You have two ground layers and one top layer.
Every layer has w x h tiles, where w is your map width, and h is your map height.
Every tile on every layer of the map has an id, which corresponds to the ids in the tileset. When you draw a tile, the id of that tile from the tileset gets saved into the id of the tile on the layer, on the map, where you draw it. The default tile id, in all the places where you
haven't drawn a tile, is 0. Remember what tile 0 is? The first tile in the B tileset! So every place where you don't draw a tile uses the first B tile by default. This is why it's always blank (in fact, even if you put a sprite there, it won't show on the map or in the tileset tab).
Now, setting the passage of a tile to O means you can walk on it, and setting it to X means you can't walk on it. So if you set the first B tile to O, you will be able to walk all over the map - wherever you
haven't drawn another tile onto the top layer. If you set it to X, you won't be able to walk on
any tile where the top layer is empty.
Setting the passage of a tile to star means to ignore the passage setting for that tile, for that layer, and look at the passage settings for the layers underneath. So setting the first B tile to a star is saying, "if I haven't drawn a top layer tile here, look at the passage settings on the ground layers".
Slightly related is that any tile with a star will also appear
above the player. This particular tile does it as well, but as there's no sprite, you can't actually see anything.