The prepositional case is used after prepositions в (in) or на (on, at).
Я в банке (I'm at the bank)
Журнал на столе (Magazine is on the table)
If the word after the preposition “в” starts with в- or ф- and then a consonant, the letter -o should be added to the preposition:
Very often English speaking students have problems with finding the difference between "location" and "direction". This is very important because with locations, the prepositional case is used. And with directions, the accusative case