The English staple comes from the Old German stapulaz (“pillar”) — from which we also get the Dutch stapel and then the Spanish etapa!
But how did a word meaning “pillar” become “stage” or “staple”? Well, a pillar holds up the next level — the next stage! (Think of floors in a building as being stages of development. Ultimately we reach the penthouse!). Or think about the pillar — that which holds everything else up so it doesn’t fall — is the staple of the building, the most basic building block, to ensure it doesn’t collapse!
We can see the t-p root in both the English and Spanish words.