It's a verbatim string indicator. Refer to the cheatsheet here. When dealing with a string that needs slashes (which are normally the start of an escape sequence) it can be hard to understand the string.
\\MyPath\\Test.txt is harder to read than
\MyPath\Test.tx. But since slash starts an escape sequence the parser sees
\M specially and doesn't know how to handle it. So for this kind of string preceding it with @ tells the parser to ignore escape sequences. Hence
@"\MyPath\Test.txt translates to
Use verbatim strings for string literals containing slashes that are not escape sequences.