to stand out is to be conspicuous, or to be most obvious or most notable, especially in comparison with other things that are similar. to stand tall is to be proud and sure of one's position or opinion.

I will expand just a little on George's explanation with some examples. stand out = easily noticed, usually when you are comparing to other things and what to show that it is significantly better, worse, or different - Philip always stands out from the crowd because of his charming personality and kind attitude. - What are some things that stood out to you about this article? - Ask good questions in your interview, so that you will stand out from all of the other applicants. stand tall = straighten yourself to stand as tall as you possibly can, to be proud of yourself or brave, not avoiding dangerous or uncomfortable situations - He stood tall, with his head high, even as they criticized the decision he made. - The coach challenged his football players to stand tall before the powerful, opposing team. - With the gay pride movement, people in the gay community are standing taller than they ever did before. I hope this helps! Let me know if you have more questions about my answer. ~ Mrs. P.