They just mean that the preceding word historically used to end in an n or m , which often survives in the Scottish Gaelic spelling, but in Irish Gaelic only only survives as a modified pronounciation of the first letter of the following word. To get at the basic dictionary headword, just strip off the initial consonant. Tha fhios agam gum bi e ann.
T fhios agam gu mbeidh s ann.
The spelling revision in Irish Gaelic about 40 years ago did away with a lot of letters in the middle or at the ends of words which were no longer pronounced. They may or may not be still pronounced in Scottish Gaelic. Here is a Scottish Gaelic - Irish Gaelic phrase list .