Slicing a 3D drawing effectively translates that 3D drawing into something that a 3D printer can understand and print.
Or, if you want a slightly more technical explanation, slicing turns digital 3D models into G-code (a generic name for a control language) that a 3D printer can understand.
Slicing software is a necessary element of 3D printing, because 3D printers cannot translate a CAD drawing by themselves. 3D printers need the specifications of the object you design to be translated into a language which they can interpret.
Basic slicing software – in fact all slicing software – will create paths for a 3D printer to follow when printing. These paths are instructions for geometry, and they tell a 3D printer what speed to print at for various points and what layer thicknesses to adopt – if applicable (sometimes it is best to do this manually). More advanced slicing programs also take into account GD&T(Geometric Dimensioning and Tolerancing). With this, it is possible to not only create slicer information about the geometry of a part, but to create information about that part’s design intent so that the finished part is suitable for longer-term end-use.