A palindrome reads the same forwards and backwards: level, racecar, 12321, or the phrase A man, a plan, a canal: Panama. With the box ticked, the checker removes spaces, punctuation and capital letters (and accents) before comparing, which is how phrase palindromes are normally judged. Untick it for a strict, character-for-character check.
Yes. A single character, and even an empty string, reads the same in both directions.
Yes. 121 and 1221 are palindromic numbers, and so is any number whose digits mirror.
See also: reverse text · anagram checker · word counter