A semordnilap is a special type of anagram. It is a word that, when spelt backwards, gives you another word. For instance if you reverse the letters in the word "strap", you get "parts". This means that "strap" is a semordnilap. Other examples in English are "desserts"/"stressed", "Evian"/"naive", and "no"/"on".
The idea was created by that master of riddle and wordplay, Lewis Carroll. As you may notice, it is "palindromes" spelt backwards. Presumably this is Lewis Carroll's version of the old complaint that the word "palindrome" is not a palindrome. This type of anagram is also known to word puzzlers as an anadrome, and was formerly sometimes called an antidrome.