The term "good shape" is often used in a few different contexts, and it is meant to suggest that something or someone is in a positive overall condition. Many people use the term to describe a person’s physical health, which could be in relation to serious health conditions or simply cosmetic issues related to body conditioning. The other most common use is to describe the condition of an inanimate object, like a piece of equipment or a house.
If a person is overweight and eventually loses a few pounds or kilograms, that person might say that he’s in better shape or good shape, and a person might also use the term after a positive doctor’s report on a checkup. The idiom can be used to describe a lack of serious health issues, an improvement that makes someone look better physically, or a general sense of well-being. For example, someone could say, "I felt bad last month, but I seem to be in pretty good shape now," to suggest that his overall health condition had improved.
The other common use for the term "good shape" is in the context of some sort of inanimate object. If someone owns a car, and it has no serious mechanical troubles, he might say that it’s in good shape, or if it’s nearly perfect, he might even say that it’s in "great" shape. Any sort of object which often needs maintenance or is notably subject to deterioration can also work well with the term, including things like houses or collectables like baseball cards. For example, if someone were buying an antique table, its price might be a lot higher if it was in good shape, and the buyer would probably be very concerned about the overall condition of the object.
The term is often used in a relative sense with a lot of subjectivity, and it doesn’t necessarily suggest any kind of exact level of quality. If something is older, people might describe it as being in good or great shape even if it has a lot of serious damage and needs a lot of work. Different people may also have very different ideas about what "good shape" means in relation to a particular subject. For example, some people may describe themselves as being in good shape simply because they aren’t suffering from a major illness, while others would only use the term if they were in top athletic condition.