Learn Spanish (Spain): ¿Tienes una goma?

The verb tienes on its own means "you have". Spanish doesn't use helper words like "do" to make questions. Also, because the verb changes its ending to match the person doing the action, the "you" is already built into tienes, so you don't usually need to add the Spanish word for you, which is tú.

How do I know this is a question if the words are in the exact same order as a normal statement?

In writing, the upside-down question mark (¿) at the beginning is your first clue. When speaking, you just raise the pitch of your voice at the end of the sentence, exactly like asking "You have a rubber?" in English.

Why is the word for "a" written as una instead of un here?

The word goma is a feminine noun, which is very common for Spanish words ending in the letter a. The word for "a" or "an" must match the gender of the noun it belongs to, so we use the feminine una to match goma.

Does goma mean an eraser here?

Yes! In Spain, goma is the standard, everyday word for a pencil eraser (it is short for goma de borrar). The English translation provided uses "rubber" because that is the British English word for an eraser.