What does this command do?

The command tail -c1 /tmp/hello does the following:

• tail: This command is used to display the end of a file.
• -c1: This option tells tail to output the last 1 byte of the specified file.
• /tmp/hello: This is the path to the file you want to read.

So, this command will display the last byte of the file located at /tmp/hello. If the file is empty, it will not output anything.