Learn Italian: I negozi sono chiusi oggi, ma questi sono aperti. - The shops are closed today, but these are open.

Questi means "these". It is acting as a pronoun here, which means it takes the place of a noun so we don't have to repeat it. It stands in for questi negozi (these shops).

Why do we use questi instead of queste?

Because it refers back to negozi (shops), which is a masculine plural noun. If we were talking about a feminine plural noun, we would use queste instead.

Why do the words for "closed" (chiusi) and "open" (aperti) both end in -i?

Adjectives in Italian must match the noun they describe. Since negozi is masculine plural, the singular adjectives chiuso and aperto change their ending to -i to become chiusi and aperti.

What is the singular form of negozi?

The singular is il negozio (the shop). Normally, masculine nouns ending in -o change to -i in the plural. Because negozio already has an i right before the o, we just drop the o to get negozi rather than writing a double "i".