Duktig means skilled, capable, or hard-working. In phrases like Oj, vad duktig du är! (Wow, how skilled you are!) it's a compliment.
You can use it on its own, in the same way as you might say in English 'you're such a hard-worker!' or (to a child) 'good girl/boy!'. But it usually refers to an action of some sort, whether that's a job or a specific task.
It is often used before a noun to highlight that someone is skilled in a particular role or capacity, so you could say: hon/han är en duktig målvakt/författare/pappa/arbetare (he/she is a good goalkeeper/author/dad/worker).
Or you can combine it with the preposition på and a verb or noun to praise specific duktighet (talent/skill): vad duktig du är på att rita/på svenska (how good you are at drawing/at Swedish). And as well as describing the person, you can also talk about the duktig things they've done: hon gjorde många duktiga saker (she did lots of good things), or use it as an adverb by adding 't': han spelade duktigt (he played well).
Duktig comes from the verb duga, which means 'to be good for/to be acceptable for'.
But the word is not without controversy. Some people argue that the word can be patronising when used about an adult, since it is best translated as “aren't you good!” and is most often associated with parents or teachers talking to children (or even pet-owners talking to their dogs), rather than peer to peer conversations. So it's a word where you need to pay close attention to context.
With that said, if someone tells you you're jätteduktig på svenska, it is in all likelihood a sincere compliment.
De vill locka duktiga personal till företaget
They want to attract capable staff to the company