If the system reads in a string, and ends up in an acceptable final state, then the string is said to be legal. If the accepted final state is not amongst the set of legal states, then the input is classified as illegal.
you can identify whether the given string accepted by FA or not by parsing the string over the FA.
for example.
given FA only accepts binary numbers that are multiples of 3. if you give 1000 thats is equivalent to 8 in decimal will not accepted by this FA so it is not legal string on the other hand if you have 1001 = (9)10 is legal
