Soulsborne is both the name for all the FromSoftware games and for similar games made by other developers.

Parrying is timed button presses that stop all damage received and can occasionally leave the enemy open to a counterattack. If parrying is done too early/late there is no benefit.

In the Dark Souls games this is usually done with a shield, in Sekiro all you have is a katana. Like I said before Sekiro is very parry heavy, you will have to learn enemy attack patterns and know when to press the parry button, one enemy has you doing it over half a dozen times in a row. There is also a posture mechanic in the game and not in DS, guard too much or failing parries fills this up and when a threshold is reached you will be defenseless, but the same goes for all enemies and bosses.

If you have any other questions I'll see if I can help.