Real talk, "master" refers to a "master copy" from which new branches ~~are~~ can be made from. If one of these branches becomes "better" than the current master copy, it gets "merged" into the master copy effectively replacing it as the new master copy (sometimes totally, sometimes partially).
Edit: Pretty sure basically everyone here knows this, but I figured it could not hurt to put the explanation here.
Real talk, "master" refers to a "master copy" from which new branches ~~are~~ can be made from. If one of these branches becomes "better" than the current master copy, it gets "merged" into the master copy effectively replacing it as the new master copy (sometimes totally, sometimes partially).
Edit: Pretty sure basically everyone here knows this, but I figured it could not hurt to put the explanation here.