This is the 3882nd most frequent Russian word.
бога
"Бога" means "God" (genitive case) in English.
In this sentence, 'Богу' is the dative case of 'бог'. It is used as the indirect object, indicating the one to whom the action of praying is directed.
Они молились Богу, прося спокойствия.
They prayed to God, asking for peace.
Here, 'бога' is in the genitive case and denotes the object of belief, expressing a sense of intangible existence.
Человек верил в существование бога.
The person believed in the existence of a god.
In this example, 'бога' is also in the genitive case and used as the object of the preposition 'в', which indicates possession or relation.