Glossary
Mid
In one line: The middle of the map, linking the sites and letting you move between them faster.
What it means
Mid is the center of the map. It almost always links the sites to each other and to the spawn areas, so whoever controls it can move faster than everyone else and threaten more places.
For the attack, taking mid means choosing the site at the last moment and splitting the defense. For the defense, holding mid means seeing the attack shift and shortening rotations. That is why on many maps the fight for mid decides the round before anyone enters a site.
An example
On Ascent the attackers take mid all the way to Market. From there they can reach B in seconds, without the B defenders seeing them come. The defense has to respond, or B falls from behind.