"I am eating rice."
Translation:Ich esse Reis.
Because we're not talking about a specific quantity of rice that the listener knows about -- we're not saying "I am eating THE rice" (the portion that we spoke about before) but just "I am eating rice": an indefinite quantity.
In English, we could also say "I am eating some rice.", since "some" is a kind of indefinite article for mass nouns and plurals, but German has no indefinite article for those cases and simply omits the article entirely. Thus: Ich esse Reis.