If you hold a valid M-card, you can enter Belgium without any formalities, providing your M-card remains valid. If you have requested an M-card but are still only in possession of an Annex 56 (which proves you applied for the M-card), you can also enter Belgium without any formalities and may work without further work authorisation requirements.
Your M-card is not a travel document; therefore, you should also always carry a valid passport when travelling outside Belgium. The passport must be valid for at least three months from the intended departure date from the Schengen Area (ideally six months - as this can help to avoid questions or delays at the border) and must have been issued within the last 10 years.
You can enter other Schengen area countries without a visa for a total of 90 days within a 180-day period. Time spent in Belgium as a holder of a valid M-card does not count toward the 90-day limit for the rest of the Schengen area.
UK citizens who held a Special-ID prior to the end of the transition period (31 December 2020) may convert to an M-card at any time. More details here.