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).
sed -i 's/\r$//g' foo.txt
The same could be done using Perl.