man bash says
pipefail
If set, the return value of a pipeline is the value of the last (rightmost) command to exit with a non-zero status, or zero if all commands in the pipeline exit successfully. This option is disabled by default.
Where “pipeline” is
command1 | command2 | command3
Without pipefail, the return value of a pipeline is the exit status of the last command in the pipeline, regardless of whether previous commands failed.
Example:
$ grep ^root /etc/passwd | cut -f 5 -d :
System Administrator
$ echo $?
0
$ grep ^nonexistant_user /etc/passwd | cut -f 5 -d :
$ echo $?
0
$ set -o pipefail
$ grep ^nonexistant_user /etc/passwd | cut -f 5 -d :
$ echo $?
1