You know when things work out even better than you hoped, and seemed to magically materialize from nothing? How things came together by no doing of your own and just fall into your lap in the best of ways? That’s "dumb luck", but I think the term itself discredits what is actually happening.
First to call something 'dumb' is simply incorrect. With the infinite intelligence that exists around us it’s naive to call anything related to your environment 'dumb'. Just because you can’t see or measure what’s happening does not mean that it doesn’t exist. There is more design and intention behind every element of your life than you realize and it’s important to respect that.
Then the word luck. I think a better word to use is "coincidence". I don't mean the traditional definition of coincidence that involves randomness, but instead the root etymological meaning. To co-incide means for things to happen together. There doesn’t need to be an element of luck or chance in what things coincide. What often happens is that we can’t perceive the synchronicity in the event, and therefore we rationalize it as a random occurrence. Again, this isn’t respecting the nature of what is actually happening in a more divine way.
In either case, if you choose to see dumb luck as a random fluke or pre-destined serendipity, it’s important to maximize the opportunity in front of you. Those who get the farthest in life do the most with the advantages they have access to. So if you want to find ways to advance your career, network, or skills, you’re going to need to take advantage of your strokes of good fortune so that they produce real outcomes.