Forms the continuous aspect, together with the gerund (which in Dutch matches the infinitive in most cases) and with zijn as auxiliary verb. Literal translation of the constructions would be "be on/at the (gerund)". In English, similar constructions, albeit without the auxiliary verb, exist but they are uncommon and marginally more frequent only in certain dialects, e.g. "I am doing the washing" or "I am doing some reading."
Ik ben aan het eten.
I am eating. (lit. I am on the eat. / I am at eating.)
Ik was aan het eten.
I was eating. (lit. I was on the eat. / I was at eating.)
Hij is aan het lezen.
He is reading. (lit. He is on the read. / He is at reading.)
Wat ben je aan het doen?
What are you doing? (lit. What are you on the do? / What are you at doing?)
Ik ben de afwas aan het doen.
I am washing up. (lit. I am the wash-up on the do. / I am at washing up.)