F1 cars are very closely matched, with a mere 1,000's of a second separating any two cars. Passing on track is difficult, so teams must look at alternative strategies. Often, this means doing the opposite of the car you are competing with.
The undercut is when a driver, struggling to get past another car, pits early to get a performance advantage from fresh tyres. Hopefully, putting them ahead when their rival pits. By getting the jump on new tyres, they can put in a series of faster laps, closing the gap and hopefully being ahead when the other driver exits the pits.
The overcut is the opposite of the undercut., where the chasing car lets the car in the front pit first. While pitting, the chasing car will push to close the gap between them in the hopes that they stay ahead even after they pit.
[caption id="attachment_8841" align="alignleft" width="1280"] Undercut vs Overcut : thesportsrush.com[/caption]