[nathankleyn]

It's what they call the releases of Chrome — M91 is version 91.

[jeffbee]

I think the number of comments here asking about the M indicate that it really is obscure jargon. The Chrome release blog doesn't use the M consistently, about:chrome doesn't use the M, and the M doesn't appear in the user agent string.

[kzrdude]

If they just had said Chrome 91 everyone would have been ok.