Multi-coloured 2 diamonds, is a contract bridge convention whereby the opening bid of 2D shows a defined range of hand types.
The strong options are also important as they may be used "garage” hand types that are otherwise awkward within your general system.
This avoids need for puppet Stayman, greatly streamlining assessment of slam prospects over a simplified 2NT opening.
While technically a "brown sticker" convention because Multi has no known "anchor suit", its popularity, and availability of published defenses, have persuaded the World Bridge Federation to allow its use even in lower rankings tournaments.
Overcalls in a major suit is more awkward to handle, but a common agreement is that opener must double or bid should he hold the strong option, clarifying type if you have 2 of those.
The "Dixon" method below is not necessarily best from a technical point of view, however it is perfectly adequate and is easy to remember.
In general, when defending against a multi players should assume that the opener holds one of the weak options.
It is helpful to discuss the concept of "position"(i.e. relative to opener) with your partner, in any defence agreement.