Names given to the Spanish language
Formally speaking, the national language of Spain, the official Spanish language, is the Castilian language (as opposed to regional Spanish languages like Galician, Catalan, Asturleonese and Basque). As such both names, "español" and "castellano", have distinct and independent meanings that may be required for clarity in some specific contexts.
Generally speaking, though, both terms can be used to refer to the (national) Spanish language as a whole, with a preference for one over the other that depends on the context or the speaker's origin. "Castellano" (as well as "Castilian" in English) has another, more restricted, meaning, relating either to the old Romance language spoken in the Kingdom of Castile in the Middle Ages, predecessor of the modern Spanish language, or to the variety of Spanish nowadays spoken in the historical region of Castile, in central Spain.
Language policies of Francoist Spain
Language politics in Francoist Spain centered on attempts in Spain under Franco to increase the dominance of the Spanish language (Castilian) over the other languages of Spain. The regime of Francisco Franco had Spanish nationalism as one of its bases. Under his dictatorship, the Spanish language (known in some parts of Spain as "castellano", i.e., "Castilian") was declared Spain's only official language.