後 [go] = after, since, later

この歌手の新しいシングルは発売後まもなく1位になりました。

kono kashu no atarashii shinguru wa hatsubai go mamonaku 1 i ni narimashita.

= The new single of this singer immediately reached the top position.

https://www.learnwitholiver.com/japanese/daily-sentence-423