MtM (Male-to-Male) is a term for transgender men who consider themselves transitioning from male to male, as they have always been male, just not in the way being male is traditionally defined as. They have always been male and will always be male regardless of whatever genital, hormonal, or chromosomal arrangements they were born with, and whether they chooses to make changes to their body or not.
It is an alterative to FtM as FtM implies that one was female at some point, then transitioned to male, which does not accurately describe how some trans men view their gender. Female-to-male is a framing that’s easier to explain, but only because it's still rooted in the concept of binary sex distinctions and assuming gender based on sex, which many trans people want to do away with entirely. The female counterpart is FtF