Busco a alguien que hable francés.
English: I am looking for someone who speaks French.
The subjunctive in Spanish adjective (relative) clauses is used when describing something that is unknown, uncertain, or desired. These are clauses that describe a noun, usually introduced by 'que'.
Use the subjunctive in adjective clauses when talking about something that may not exist, is hypothetical, or is not specific. For example, when you want, look for, or need something but are not sure if it exists.
Busco a alguien que hable francés.
English: I am looking for someone who speaks French.
No conozco a nadie que viva aquí.
English: I don't know anyone who lives here.
Quiero un coche que sea rápido.
English: I want a car that is fast.
¿Hay un restaurante que sirva comida mexicana?
English: Is there a restaurant that serves Mexican food?