An equestrian is an expert horseback rider. If you only go riding for an hour while on vacation you aren't an equestrian, but the person leading the group and riding the flashy horse probably is.
When you know that equus is the Latin word for "horse," the meaning of equestrian becomes clear. An equestrian is someone who is involved with horses. You can also use it as an adjective to describe anything having to do with horseback riding. An aspiring equestrian may have to give up socializing with friends in favor of equestrian duties like grooming horses.