Things that are uncountable are too numerous to be calculated or added up, like the uncountable stars in the sky.
Some things are clearly uncountable, like the infinite digits in the number pi or the blades of grass in your town park. There's literally no way you could possibly count them all. Other things could theoretically be tallied, but they're still uncountable — at least metaphorically. If you're totally overwhelmed by a huge surprise birthday party, the crowds of guests might seem uncountable, leaving you longing for a quiet celebration with your two best friends.