The following two characters are used to represent a carriage return.
Let's say foo.txt is transferred from a Windows system to a Linux system. Windows will end each line with a new line (\n) and carriage return (\r). On Linux, lines only end with a new line (\n), thus sometimes you may want or need to remove the carriage return (\r) from each line.
This command will remove the carriage returns (\r).
perl -i -pe 's/\r$//g;' foo.txt
The same can be done on Linux using the sed command.