Example without escape slash (as text, would break if run):

$quote = "She said, "hi" today";

Example with escape slash:

She said, "hi" today

The escape slash is a backslash placed before a character so PHP treats it as plain text instead of code. In the second example the backslash lets the quote marks show up inside the string without ending it early. Without the escape slash, like in the first example, PHP would misread the extra quotes as the end of the string and cause an error.