The prepositions a, da, di, in, and su, when used with a definite article, combine with the definite article to form one word. These are often referred to as prepositional articles, contractions, or articulated prepositions.
Vado a+il negozio = Vado al negozio. I am going to the store.
Sono i bagagli di+gli studenti = Sono i bagagli degli studenti. They are the students' suitcases.