"I chose the red boots."
Translation:J'ai choisi les bottes rouges.
Des would mean some. Les means the when we would use the, and means all of the ... in the world when we wouldn't use anything. The English sentence infers that a specific pair of boots are already known within the conversation and so les is appropriate, but if the red boots were just being introduced to the conversation the English would be "I chose some red boots" and the French would use des. I hope that makes sense!