That is the version of the local conditions report which they received. It used to be "Information xxxxx" where xxxxx was the version letter.
As they approach the airport the pilots will tune to the ATIS channel, which continuously loops a brief report on local weather conditions, runway status, hazards, etc. When the airport updates their report they will change it from "Information Alpha" to "Information Bravo" or whatever is next. The pilots announcing that they have "Information Mike" lets the controller know they received the current report.
Airnav lists EWR's ATIS channels as 115.7 and 134.825. You wouldn't want to scan them since they're always going, but you may want to program them and lock them out.