This is a topic that really interests me because there are different opinions everywhere. Some say it was effective at stopping slashes but not stabs, while other say it protected against both types of attacks. Some say it was effective against projectiles like arrows, some say it wasn´t.
/u/WARitter has previously answered:
See also AskHistorians Podcast Episode #82 and AskHistorians Podcast Episode #83
/u/Valkine has previously answered Did Medieval armor and weapons have "specs", like we have for cars or computers? Did Knights or other fighters compare specs when choosing what to use, and discuss their "builds" with their Knight friends?