A regional theater company that only produces a season in the summer time.
Traditionally, summer stock companies performed shows in repertory (stock being a synonym for repetory or repetoire) offering several plays in one season, in rotation. Such a company would rehearse several shows at once so that performances can change every night, and an actor who is a leading man one night might find himself in the chorus the next and playing a sidekick the night after that. The abbreviated season makes for a condensed rehearsal period, and a technical crew that is pushed to its limits.
Broadway and regional theatres in urban centers used to end their theatre season by June, freeing up actors for summer work "out of town." Summer stock companies also take advantage of college students available to perform. Many summer stock companies began in tourist or vacation areas, producing light comedies and musicals on the assumption that folks on vacation don't want to have to think, thus leaving the dramas for the year round companies. While many companies do produce musicals and old favorites in the summer, the rise of Shakespeare, Shaw, and Fringe Festivals in the summer months means that you're likely to find just about any type of show in the summer.