They're a commit with two parents, one of which has no real meaning anymore because it doesn't exist anywhere else. They break a lot of tools because of this.
The second parent is the last commit of a series of commits starting at a commit, sometime in the past on the main branch, followed by all the new commits that represent the feature that is being merged.
Merges are horrible, if you don't have a mental model of git that aligns exactly to the above.
They're a commit with two parents, one of which has no real meaning anymore because it doesn't exist anywhere else. They break a lot of tools because of this.