A critical component of swimming strokes is the catch, the point in the pull at which you begin forward propulsion. A rule of thumb for many coaches regarding the catch is “the earlier, the better” and this is the basis for the common admonition against dropping your elbows in the pull. The video below from GoSwim demonstrates a drill for practicing a good catch in freestyle.
An early catch is sometimes called an Early Vertical Forearm to emphasize the importance of a high elbow position. There are a number of threads (such as this one) in the USMS Discussion Forums that discuss this and other important issues of swimming technique.