Design Tokens
Design tokens are named values for design decisions such as colours, type sizes, spacing, corner radii and shadow levels, defined centrally and used identically in design tools and in code. They are the technical translation of brand guidelines into machine-readable values.
In practice
The advantage becomes obvious with the first change request: alter the brand colour in one place and it changes on every button, link and chart, instead of having to be hunted down across thirty files. As a client the relevant question is not what tokens are called, but whether colour and spacing values are maintained centrally or scattered through the code. Sensibly named tokens describe purpose rather than appearance: a value called "action colour" survives a rebrand, one called "blue" does not. Tokens are also the clean basis for a light and a dark mode. At handover, have it confirmed that these values are documented and changeable without the agency.