Something that's infrequent doesn't happen very often. Your family might take infrequent trips to Disney World, traveling there every two or three years.
Things that are infrequent are rare or uncommon. You'd probably be glad to have infrequent run-ins with mean dogs or ugly insects, but disappointed to get infrequent phone calls from your best friend. The adjective infrequent comes from the prefix in, which here means "not," and frequent, or "often," which is rooted in the Latin word frequentem, "crowded or repeated."