Have a look at the following:
The cow who laughs
The man whom I love
The woman whose coat I like
The place to which I drive
The roads on which I walk
In these sentences the red words and phrases are examples of the relative pronoun.
English is a bit lazy with it. There is a clear distinction between who and whom, for example, but it isn’t always followed. Sometimes neither is used. “The man I love” can work fine without ‘whom’. But it is worth looking at the formal style of English. It will makes the Latin much easier to grasp.
Here we see the relative pronoun in five different cases. We have nominative (the cow is the one laughing), accusative (the man is being loved, he is not the one loving), genitive (the coat is the woman’s), dative (driving to somewhere) and ablative (walking on something). So you can see that the relative pronoun plays an important role in joining bits of sentences together into larger units.
English doesn’t distinguish between singular and plural versions of these pronouns. You can say “the road on which I walk” or “the roads on which I walk.” The nouns changes from singular to plural, but the pronoun (on which) stays the same. We do make some distinction for gender, as least between people and things. Formal English says “the man whom I love” but “the book which I love.”
Now let’s see how the relative pronoun operates in Latin.