This page of the Elden Ring game guide explains how to parry correctly, i.e. reflect enemy attacks aimed at you. Additionally, you will find out if using this mechanic is important in the game.
Parrying - is it necessary?
It's worth starting with the fact that parrying is absolutely not necessary to deal with opponents easier and faster. First of all, you won't always be able to parry an opponent, as some attacks cannot be countered. Other attacks can be so fast or hard to predict that even though they can be parried, it's not really worth doing. The reward for parrying such an attack will be much lower than the risk of taking significant damage.
Secondly, parrying will also not be necessary, or sometimes even available, depending on what build you choose. If you decide to play as a mage or a ranged character, you are unlikely to carry a shield in your inventory. It's true that certain one- or two-handed weapons sometimes give you the option to parry, but there aren't many weapons like this. Also, the parrying animation is slightly different compared to the shields, and sometimes confusing.
To sum up, parrying is still a very useful mechanic that's worth learning, assuming that your build is centred around it. Knowing how to parry is useful in many situations, but it isn't required to complete the game.
Parrying, i.e. deflecting enemy attacks, is a high risk, high reward mechanic. You are risking a lot by using this technique. If you manage to pull it off correctly, the payoff is really big.
In order to learn how to parry correctly, you should practice this mechanic on an enemy that won't deal too much damage to you and whose attacks are easy to read.
Once you have found such an enemy, provoke them to attack you. However, don't try to parry the first hit. Instead, block their attack or move away to avoid taking any damage. Observe their attack animation. You should parry when the enemy's weapon is right in front of you, not at the very start of the attack.
You need to parry when the enemy's weapon comes close to your character. If you do this correctly, you will hear the iconic sound of a parried attack. In addition, the enemy will be thrown off balance for a short while, giving you the opportunity to approach them and then counterattack to deal tons of damage.
If you fail to parry, you will see that the opponent is still attacking and you take damage. Usually, the enemies will chain their attacks, so it's not worth trying to deflect the hit again after the first unsuccessful parry, as the series of attacks will continue. The main reasons are that you most likely won't be familiar with the animations and there's a chance you might die after taking two hits in a row.
Timing is critical, as it determines whether you have performed the parry succesfully. If you do them too early or too slow, you won't block the attack. When the enemy attacks and you notice them swinging their sword or other weapon, wait a short moment and then parry once you see that the weapon is nearly touching you.