What does M-NTC mean when used with UFAs?

NTC = No Trade Clause. Which means a player can't be traded without his consent. Without the "M" it's considered a full-NTC.
M=Modified. Which means there are stipulations where a player can be traded without his consent. It usually involves making list of a certain number of teams where he will or won't be traded.

NMC=No Movement Clause. Which means a player can't be sent to the AHL or traded (full NTC) without consent. If the M-NTC is with it it just means he can't be sent to the AHL and can't be traded to certain teams.