The meaning of the preposition в/во depends on the case of its object. If the object is Accusative, the preposition means 'to, into'; if the object is Prepositional, the preposition means 'at, in'.
в plus Accusative means motion to, into.
в plus Prepositional means location at, in.
Motion (Acc.): Ћн ходќл в Большћй теђтр. He went to the Bolshoi Theater.
Location (Prep.): Ћн бІл в Большћм теђтре. He was at the Bolshoi Theater.
Motion (Acc.): Ћн ходќл в университљтскую библиотљку. He went to the university library.
Location (Prep.): Ћн бІл в университљтской библиотљке. He was at the university library.
Motion (Acc.): Ћн ходќл в пђрк. He went to the park.
Location (Prep.): Ћн бІл в пђрке. He was in the park.
The variant во is used before consonant clusters beginning with в-, ф-, and мн- as well as before certain vowel-less stems, e.g. во что, во сне. Similar variations occur with several other prepositions, e.g. as к/ко and с/со. The entry in 5000 Russian Words or The Russian Dictionary Tree will tell you which consonant clusters affect each preposition.