Q: What do you think this is and what can I do?

A: It looks like an example of when a limb stub is left on the tree trunk. If I’m seeing it correctly rainwater has invaded through a rotten limb opening. It gradually came inside the tree trunk, which rotted.

Eventually the rot literally broke through the bark and became the long void that you see.

The tree bark near the wound tried to grow over the void but was unsuccessful. If the tree is not near anything important, you could let it live. The wound will probably never close but the trunk will become marginally stronger over time.

Or you could be pro-active and cut it down now.