Relative clauses are used to refer to something mentioned earlier.
a) The man that I see is blond.
b) The man, whom I see is dark.
c) The thing that makes everything complicated is quite small.
d) I was unhappy which made things even more difficult.
As you can see there are several relative pronouns in English. In English there is a difference between the function of the relative pronoun (to be explanatory or determining) and whether the referred subject/object is a living person or not. In French it is even more simple. There is the differenciation only according to the kind of object.
subject: Les hommes qui traversent la rue sont blonds.
The men who cross the street are blond.
subject: La femme qui traverse la rue est blonde.
The woman who cross the street is blond.
subject: Les femmes qui traversent la rue sont blondes.
The women who cross the street are blond.
direct object : La femme que je vois est blonde.
The woman whom I see is blond.
direct object : Les hommes que je vois sont blonds.
The men whom I see are blond.
direct object : Les femmes que je vois sont blondes.
The women whom I see are blond.
As you can see the relative pronoun qui is used as a subject of the relative clause. The relative pronoun que is used for a direct object in a relative clause.
Ok, we have left out the indirect object, but we will come back to this later in chapter 11. For now this is a good start and you will be understood. Then we will also come back to the issue of prepositions and relative pronouns.