Nouns ending in -on are usually masculine.
Words ending in -tion, -sion, -son, or -aison follow different rules.
Is bon masculine or feminine?
lebon
The rule behind it
91.9% of -on nouns are masculine
In context
Je vais arrêter de fumer pour de bon.
I will quit smoking for good.
J'espère que tu as fait bon voyage.
I hope you had a nice trip.
Il a l'air de faire bon dehors.
It seems warm outside.