Fit refers to the mating of two mechanical components. Manufactured parts are very frequently required to mate with one another. They may be designed to slide freely against one another or they may be designed to bind together to form a single unit. The most common fit found in the machine shop is that of a shaft in a hole.
There are three general categories of fits: 1) Clearance fits for when it may be desirable for the shaft to rotate or slide freely within the hole, this is usually referred to as a "sliding fit." 2) Interference fits for when it is desirable for the shaft to be securely held within the hole, this is usually referred to as an interference fit and 3) Transition fits for when it is desirable that the shaft to be held securely, yet not so securely that it cannot be disassembled, this is usually referred to as a Location or Transition fit.
Within each category of fit there are several classes ranging from high precision and narrow tolerance (allowance) to lower precision and wider tolerance. The choice of fit is dictated first by the use and secondly by the manufacturability of the parts.