Learn Italian: Molta gente crede all'oroscopo, ma io non ci credo. - A lot of people believe in the horoscope, but I don't believe in it.

While ci often means "there" (referring to a place), it also acts as a pronoun that replaces a phrase starting with the preposition a. Because the first half of the sentence uses credere a (crede all'oroscopo), we use ci in the second half to replace all'oroscopo. Here, it translates perfectly to "in it".

Why is the verb crede singular when "a lot of people" is plural?

In Italian, the word for "people" is gente, which is a feminine singular noun. Even though it represents a group of individuals (a plural idea in English), all the words that agree with it must be singular. That's why we say molta gente instead of molte and use the singular verb crede instead of the plural credono.

Why does the sentence use all'oroscopo instead of nell'oroscopo to mean "in the horoscope"?

English uses "believe in", but Italian often uses the verb credere followed by the preposition a when talking about believing in abstract concepts, stories, or things like the horoscope (credere a qualcosa). All' is just the combination of a + l' (the). Note: Italian does use *in when believing in God (credere in Dio) or in yourself, but uses a for most other things.*