what is the meaning of this simple command

It isn't a command.
It is a single character (hence the single quote characters) which is a single backslash character.
In strings and character literals, in order to enter special characters which can't be typed or which might be confusing, you use a special character as a sequence introducer - the backslash character, and that telkls the compiler to take teh next character as a special
character rather than interpret it as is.

For example, if you want to enter a single quote character in a character literal, it is not that obvious what ''' means: so you enter it
as

'\''

'   Start of character lieteral
\   Start of special character sequence
'   Insert a single quote character
'   End of character literal

To insert a backslash character, you prefix it with another backslash character as the sequence introducer.

Common sequences include:

\\   Back slash
\'   Single quote
\"   Double quote
\n   Newline
\r   Return
\t   Tab