Learn Spanish (Latin America): La señora compra unas sillas.

unas is the plural form of the feminine article una. Because the noun sillas (chairs) is plural and feminine, the article must match it. Together, unas sillas translates to "some chairs".

Is it absolutely necessary to use unas in this sentence?

Not strictly! Just like in English, you could say La señora compra sillas (The lady buys chairs). Using unas changes the meaning slightly to "some chairs" or "a few chairs", making it sound like a specific, but unstated, amount.

Why does the verb end in "-a" (compra) here?

The verb is comprar (to buy). Because the subject of the sentence is La señora (she), we use the third-person singular form, which drops the "-ar" and adds "-a" to make compra. If you wanted to say "I buy", you would use compro.

I've seen señora used for "Mrs.", but here it means "lady"?

Yes! When placed directly before a person's last name, señora means "Mrs." But when used on its own with an article like la, it simply means "the lady" or "the adult woman".