If someone or something is described as being "in the swing," it means that they are making good progress at whatever they are doing. This is an English idiom that is used whenever someone gets into a good rhythm doing something, especially after a rocky start. The idiom is very similar to other idioms like "in full swing" or "in the swing of things." It most likely evolved from "in full swing," which is a reference to the sport of tennis when someone is making a complete stroke to put a got hit on the ball.
Whenever someone uses a word or phrase that has a meaning in the culture that is accepted as different than either its original intent or the literal definitions of the included words, he or she is using an idiom. Idioms are important parts of a person's daily speech patterns because they allow for things to be said in a colorful and colloquial manner. They often can provide a type of shorthand to speakers as well. Many of these idioms are derived from the world of sports, such as the expression "in the swing."
Anyone or anything described in this manner has reached some level of equilibrium in whatever task is engaged. There is a sense of smoothness associated with the phrase as well, as if things are progressing in good order. Individual persons or entire groups of people can be described in this manner. For example, someone might say, "I really feel like I'm in the swing at work lately since I have been getting so much done."
In many cases, there is an implication with this phrase that the current period of things running smoothly may be following up a previous rough patch. This idiomatic expression is commonly used to describe someone who may have struggled when first taking on a task but, after that, has found a level of comfort and has been making impressive progress. As an example, consider the sentence, "I doubted whether he was right for the job, but after that bad start, he is really in the swing."
The sports world, with its rich terminology, is the source for many idioms. "In full swing" is a tennis term referring to a player who is fully extended to make a clean strike of the tennis ball. It's likely that this phrase was the source for "in the swing" and other phrases closely related to it.