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

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

What does O(1) time complexity mean?

What does O(n) time complexity mean?

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

Which of the following is the best time complexity?

What does O(log n) time complexity mean?

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