What does O(n^2) time complexity mean?

What does O(1) time complexity mean?

What does it mean when an algorithm has a time complexity of O(1)?

What does it mean when an algorithm has a space complexity of O(n)?

What does O(n) time complexity mean?

What does O(log n) time complexity mean?

Which of the following is the best time complexity?

In the context of algorithm complexity, what does the term 'best-case' refer to?