Avagamanam is derived from the verb gam (1 P) “to go” by addition of the prefix ava and suffix ana. The resultant substantive avagamana is an abstract noun in neuter gender. For use in language it must be a word, and it is avagamanam. The prefix ava does not change the meaning of the verb. Hence the substantive means “Knowledge”.
Technical: ava + gam + lyuṭ = ava + gam + ana = avagamana. Note that the entire suffix is coded, the first and last letters do not result in any change that is not relevant for our purpose here, yu on the other hand is a code for ana, the suffix that is actually added. The notation (1, P) means the verb belongs to class 1 and P stands for parasmaipada. These are relevant only for conjugation of verbs to yield finite verbs and formation of present participles.