This is the 4581st most frequent Russian word.
гостях
"Гостях" means "being a guest" or "visiting someone."
The phrase "в гостях" (in addition to "у друга" which provides additional specification) indicates being at someone's place as a guest.
Вчера я был в гостях у друга.
Yesterday, I was visiting a friend.
The phrase "в гостях" conveys the state of visiting someone's home ("у бабушки") with a connotation of hospitality.
Мне очень понравилось быть в гостях у бабушки.
I really enjoyed visiting my grandma.
The usage of "в гостях" here emphasizes the experience of being a guest at one's friends' place ("у своих друзей").