Many people believe that NFL wide receivers do not perform their best until their third year in the league. Their third year is considered to be their breakout year. Most people say that the reason for this is that receivers need to gain experience in the NFL before they can succeed.
Jokingly, some people suggest that the NFL stands for Not For Long. That is because most players who are in the NFL only last in the league for a few years. Running backs especially do not last very long in the league, as there is a tremendous physical toll on their body.
Wide receivers certainly get knocked around a lot, but not as much as much as running backs. This allows receivers to stay in the league for longer. Since they don't get injured as easily, wide receivers actually benefit from having more time in the league. It allows them to gain experience and sharpen their skills.
The football community is fairly split about how real the 3rd Year WR rule is. There is certainly some truth to the rule: wide receivers do often have their best years in their third NFL year. With that said, that doesn't mean you should always avoid players who aren't in their third year. It is wise to be cautious about rookie receivers, but you can also take a chance on non third year receivers if they seem like a good draft pick.
Third year receivers often have a high draft stock because of the high expectations surrounding a third year players. Be wary not to draft a receiver too early just because it's their third year in the NFL.