Most people think through their decisions by evaluating what the immediate outcome will be.
"I'm late," might evaluate to, "I should drive faster." Simple enough, right?
That's first-order thinking. Evaluating the immediate outcomes and basing your decision on that. Naturally, second-order thinking goes one step further and asks what will happen after the first immediate outcome.
"If I drive over the speed limit, what might happen?" Well, you'll be speeding, so the chances of getting either pulled over or in an accident are both much higher. Therefore, you should pay more attention to cops and be more aware of your surroundings.
This is a trite example, of course, but the framework stays the same for every decision. Second-order thinkers always ask, "And then what?"
Some life decisions have seemingly negative first-order outcomes, but something positive lies within the second and third-order. And on the contrary, some decisions have seemingly positive first-order outcomes, but something negative lies in the second and third-order outcomes.