Pattern matching is finding if a certain sequence of elements exists in another larger sequence of elements. For example we want to find if the string "abc" is in the string "abdabdbdacbaabcasd" (which the answer is yes).
If we have the needle string "abcxabcy" and haystack string "abcyabcxabcy" then our first search will be putting the needle at position 0 as we see the search fails at 'x'. However we note that we do not need to set the needle at position 1 because we have already done the search for the prefix "abc". Thus we can search starting by setting the needle at the next 'a'.
KMP uses this type of optimization for pattern matching by precomputing a table for the needle string.