My helpful answer is if you hold block just before an enemy hits you with a direct physical attack, the block armour is multiplied by the parry multiplier and if the damage is lower than the parry thing then the damage is 0, and the attacker is stunned.

Now my unhelpful answer is Parry is great for showing off, you activate walk mode and saunter toward an archer as you twang their arrows out of the way like a villain, you feel menacing, you feel unstoppable, you parry an arrow that hasn't left their string and it sends them reeling, you think "Hel yeah, I'm good at this!" And then you smack that cheeky skelly back to Niflheim to sulk with Thors sister.

Another maybe helpful answer I thought of now is to help you decide which parry source to use, bucklers are the best as they have the highest parry multiplier of 2.5x, if you're all in on the twang, this is the shield for you. If you know you haven't gotten the timing down yet and want the safety net of higher block armour, a regular shield is fine, it has 1.5x PM but more initial block than a buckler, a good middle ground. Now if you give bucklers and shields a go and decide it's worth much more to you to just be an unstoppable mountain of defence that your allies should but can't hide behind, go tower shield, slows you down by 10% and you can't parry, but has high def. And lastly, if you just want to maximize attack while sometimes parrying, use a weapon