Understeer is a driving condition where the vehicle doesn't turn as much as the driver would like, because the front tires are sliding. There are a number of conditions that can cause a vehicle to understeer, and many manufacturers build cars with some intentional understeer in their design because it is safer than oversteer, the opposite driving condition.
Understeer in Motorsports
Understeer is generally regarded as undesireable, but sometimes preferred to oversteer by some drivers. Understeer may be referred to as push, pushing, or tight in some forms of motorsports.
Aero-Understeer
While a vehicle may be mechanically balanced, it is possible that when enough speed is gained, aerodynamics come into play, and can change the balance towards understeer. Typically, that means downforce will be stronger at the rear than the front.
Snap-Understeer
A sudden understeer experience is considered "snap understeer", especially when it occurs unexpectedly compared to how the vehicle normally performs.
A Snap Understeer Example
In this video uploaded by Furripupau, this 1973 Saab 96 had recently been put back on the road, and was on 10 to 12 year old tires. The curve in the video is wet, off camber (meaning the road is angled downward, toward the outside of the bend), and it is tighter than 90°. A number of accidents have happened on this particular spot in the road.
If a car had been coming the other way at the wrong time, this snap understeer would have resulted in a crash. Also note that there was a little bit of oversteer at the end of the corner.