The marshmallow test is when you stick one marshmallow in front of a kid and tell them they can have this now, or, if they wait an hour, they can have two. Then you walk away.
Supposedly kids with self control who wait an hour without eating the marshmallow will have better SAT scores and a better life filled with fame and fortune, while those who greedily snatch the marshmallow will be dependent on their in-laws. I'm not sure about the veracity of the conclusions, but the experiment is a good example of time preference.
Time preference measures whether you placate your present self or optimize your future self. A low time preference means that you will sacrifice today in order to improve your future self. A high time preference means that you’ll eat junk food and let “future you” workout and diet.
Most people think that low time preference is always preferable to high time preference. The benefits of low time preference are clear, it's exercising or controlling your appetite to stay fit, it's studying to do well on a test, or "putting the time in" to get promoted at a job. But I don’t think it's universally a good thing. If you always put “future you” as a priority, “present you” loses out. Low time preference could be a justification for not taking chances, for grinding something out rather than changing your situation.
Time preference is merely the framework to look at a situation. You should know, for a particular circumstance, what you value in the short term and long term, and which one you value more. Focusing on the long term is not an end in itself. You need to properly assess values for the near term and the long term and make your decision from there.