What does the left angle bracket after a while loop mean in bash?

The following is from /etc/init.d/functions on RHEL. I'm trying to figure out what the __pids_var_run() function does when I came across this while loop.

            while : ; do
                    read line
                    [ -z "$line" ] && break
                    for p in $line ; do
                            if [ -z "${p//[0-9]/}" -a -d "/proc/$p" ] ; then
                                    if [ -n "$binary" ] ; then
                                            local b=$(readlink /proc/$p/exe | sed -e 's/\s*(deleted)$//')
                                            [ "$b" != "$binary" ] && continue
                                    fi
                                    pid="$pid $p"
                            fi
                    done
            done < "$pid_file"

Could someone explain what while : ; do ; ... done < "$pid_file" does? More specifically, the last part after done, as the rest of it more or less makes sense.