To canter is to ride a horse at a speed between a trot and a gallop. When canter is a noun, it is a ride on a horse at that pace and so you can say to a friend, "Come with us for a canter!"

As a verb, canter means to ride at the pace of a canter, which is a pretty easy rate of speed. In fact, the word comes from the phrase "Canterbury gallop" because in the Middle Ages people used to travel to the cathedral in Canterbury, England to visit the tomb of St. Thomas Becket. It was so popular an activity that the pace of the horses that the pilgrims rode on became known as the "Canterbury gallop" and the phrase was so commonly used that it was eventually shortened to canter.