The first parent is (generally) main. The second parent is the last commit of a series of commits starting at a commit, sometime in the past on the main branch

The first parent is (generally) main. 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.