Signal: Terminal Input on Background Process

The process is trying to read from a terminal that it no longer is the controlling terminal for. This process will be stopped until a SIGCONT is sent. This signal can be caught and ignored, see SIGTSTP, SIGSTOP and SIGCONT

An example is
bash$ cat > /dev/null & [1? 31731 [1? + suspended (tty input) cat

By default, cat will read from standard input, but the & character means that this command is run in the background, and the terminal's input goes back to the shell. When cat tries to read from the terminal's stdin, a SIGTTIN signal is raised.