Think nothing of it, is one of the most polite ways to say "you're welcome". Let's say that you have done a big favor for someone and spent 2 hours stacking firewood outside their house. These are people you know and like very much, so it is extra kind of you to tell them, when they say thank you, "think nothing of it." However, this phrase is limited to favors and good deeds. If you hand someone the salt at the dinner table, and when they say thank you, you say "think nothing of it"; they will look at you very strangely, because you have not done them a big favor at all.

You can also say: Not a problem.