The contact patch is, as the name suggests, the area of the tyre (and hence the vehicle) that is in contact at any one time with the road surface. There are many factors that will have on the size and shape of the contact patch and these include acceleration, braking, inflation pressure, tread depth and tyre design.
A tyre with a large contact patch will generally give good dry performance grip. Conversely this tyre may not give such good wet performance as a narrower tyre due to this large contact patch: a larger contact patch will distribute the car’s weight over a larger, area increasing the risk of hydroplaning at high speed. This is often offset by the specially designed tread patterns and compounds now available.