In Microspeak, fit is a predicate noun which is never used on its own but always comes with a modifying adjective. For something to be a good fit is for something to be appropriate or suitable for a particular situation. The opposite of a good fit is not a bad fit, because that's pejorative. Rather, something that is not a good fit is referred to as a poor fit.
The purpose of a previewer plug-in is to allow users to view the media without opening it. An image editing tool would not be a good fit for the previewing feature. (Alternatively, "would be a poor fit for the previewing feature.")
To be a good fit with a particular group is to mesh well with that group's existing practices and conventions.
The Datacenter Edition of the product is a poor fit for most small businesses.
The group in question need not consist of people.
The results are obtained incrementally, which makes it a good fit for
Microsoft Human Resources loves to apply the concept of "fit" to people fitting into a job position.