Calipers are tools which are used to measure things. They consist of two legs attached to a central hinge; the legs can be moved closer together or further apart to obtain a measurement. Different calipers are designed for specific purposes, like measuring the insides of objects and the outsides of objects. Many trades require the use of these tools, and they're typically readily available from stores which cater to these trades.
A very basic set of calipers looks rather like a set of pincers. When the pincers curve inwards, as though they are going to grip something, the tool is used to measure the outsides of objects, like screws, nuts, wooden planks, and so forth. When the pincers curve outwards, it's designed to measure the insides of things, like tubes. Typically the central hinge has a set of markings which can be read to obtain distance measurements.
It is also possible to find calipers mounted to a slide rule, in which case they are moved along the ruler to obtain the desired measurement. One famous example of this design is the Vernier caliper, which has two sets of guides which allow the user to measure interiors and exteriors. Vernier calipers often come with multiple systems of measurement in their markings, allowing people to determine distances in both Imperial and Standard measurements, for example. It is also possible to find some with digital readouts which eliminate potential operator error.
Some versions of these tools are fixed open at a specific width. They are used in quality assessment and to maintain standards; for example, a company might use fixed calipers to ensure that it is producing bolts which are the same size. By contrast, movable calipers are totally adjustable, allowing people to measure a range of objects, although they can sometimes be locked into place for the purpose of standardized measurements.
Calipers are extremely useful measuring tools, and they are very easy to use. Some people find these tools rather addictive when they are first introduced to them, as these devices can be used to measure all sorts of things. In specific trades where they are used, experienced people often prefer one type or another.