A "knockout tie" refers to a sports match or competition where the losing team or individual is eliminated from the tournament. It means that if they lose this match, they're out of the running. Full definition
However, the key reference ahead of this game is Neymar's last Champions League knockout tie in this stadium. (theringer.com)
The club will look for a new coach, and they will be tempted to go for a big name after the failure of the Emery experiment — the Spaniard won three Europa Leagues with Sevilla but has never won a Champions League knockout tie in six seasons of trying. (newindianexpress.com)
The Arsenal and France international defender Laurent Koscielny is apparently fit again and available to play in the second leg of the Champions League knockout tie with La Liga giants Barcelona and hopefully our best centre back will then stay fit for the rest of the season. (justarsenal.com)