Variable Fonts technology allows users to navigate between two or more specific styles (called masters or sources) with high precision and much smaller font file sizes than several static fonts.

For example, the interpolation between Regular and Bold allows the generation of a Medium style (also called instance) as well as any other intermediate state.

This process relies on the compatibility of the outlines across masters, meaning they must share the same structure and number of points to interpolate correctly.

Interpolation allows variable fonts to have a reduced file size because they store only two masters (or more) and their variations data, rather than separate outlines for every possible style.

FONT ENGINEERING ADVICE

A variable font technically contains only one full master: the origin. This is the default style shown when variable font technology is not supported.
The other masters are stored as delta data—mathematical differences from the origin—used to interpolate and generate other instances dynamically. This delta data is stored in the gvar table.