For the most part, a seed of 0 means that your Minecraft world is generated with little to no randomness. This is a result of a bug inside Minecraft's seed parsing function. For example, typing "pollinating sandboxes" and "creashaks organzine" will produce the same seeds in Minecraft.
Aside from that, seed 0 can also be the result of an upgrade bug can could happen if you are playing in a world from Minecraft 1.8 and older and updated it to 1.20.