Breeches are an old-fashioned kind of short pants that end at the knee. These days, you're most likely to wear breeches if you ride horses (or if you like to dress up in 18th-century clothes).
Riding breeches are designed to make horseback riders comfortable and streamlined. They're snug-fitting and usually meant to be worn with tall riding boots. Fencers wear a similar type of breeches as part of their uniform. From the 16th to the 19th century, breeches were ordinary adult men's clothing, sometimes also called britches.