It usually means the molecuel lost one proton by the electron spray. So it usually is the maximum rank in the Mass spec and it shows you roughly the molecular weight minus 1.

thank you

Addendum: the "mass minus one proton" scheme applies when you use anions, for cations the protons stay on it.

You really have to watch out what your mass spectrometer does because some companies (e.g. Waters) wrote mechanisms in their software that adds the proton mass automatically even if it doesn't make any sense (because the molecule cannot be protonated or prefers other mechanisms). So, if your masses are too heavy by 1 Da, check if the tool does that.