What does it mean when my bread rises like this?

When bread splits along the side rather than breaking open on top, such as yours did, it tends to mean that it was underproofed. A good indicator of bread being ready for the next step (shaping or baking) is that you can poke it gently, and the indentation of your finger should remain. The more rapidly it springs back indicates more proofing time is needed.

Use a single edge razor blade, or a very sharp knife to cut an escape path for the rise in the oven.
You can do this one long cut down the top or a few cuts sliced on top at right angle to the axis of the bread.

Nice metallic 'tin tile' BTW.