A string constant, as you have already seen, is a sequence of characters between quotation marks. You can use either single or double quotes. Single quotes are most common, since they do not require pressing the shift key. However, if a string must contain a single quote, then it must be delimited using double quotes. If a string starts with a ', then the compiler will think the next ' is the end of the string.
print *, "You can't delimit this string with ''"