bonvenigi

Decode it and it comes apart cleanly. Bona is good, veni is to come, so bonven- is good-coming. Add -ig-, the suffix that means to cause or to make, and bonvenigi is to make someone's arrival a good one. English welcome is built from precisely the same two ideas, wil and cuma, and has simply worn down too far to show it.

The greeting is Bonvenon!, and the accusative on it is not a typo. It is an elliptical accusative, short for mi deziras al vi bonvenon, I wish you a good arrival, in the same family as Bonan tagon! and Dankon!. Learners routinely write Bonveno and it looks wrong to any Esperantist, so learn the phrase with its -n attached.

The -ig- is what stops the word from meaning the wrong thing, so it is worth checking against its partner. Bonvenigi is transitive: you welcome someone, and the object is the guest. Bonveniĝi would be to become welcome, which is something the guest does rather than the host. The whole igi and iĝi contrast lives in that one letter, and this is a tidy place to test whether you have it.