In D+D 5e, there are many effects that enable AC, attack rolls, or damage rolls to be modified after being rolled but before the hit is resolves. Examples are the Shield spell and the "Parry" Battle Master maneuver.
When the opponent makes an attack roll against you, is the DM supposed to roll and say, e.g. "does an 18 hit your AC", or are you supposed to tell the DM your AC in advance and he just tells you if it hit or not? This is relevant because in the first case, you will always know if a Shield spell (for instance) would avoid the attack.
Same thing with damage rolls – do you know how much damage was rolled before you decide whether to use a Parry?
Best Answer
Shield specifically specifies that you are "hit" with an attack. Parry specifies that you are damaged.
Generally things that allow you to use your reaction will proc on one of several conditions:
With each of these the circumstances of the player DM interaction is slightly different.
With the first, the DM says "The monster targets you with X" and you say "ok, wait, I'm going to use X" before he rolls the attack.
With the second, the DM says "The monster targets you with X, his roll is 20, does that hit your AC?", you say "It would, but I'm going to use Shield, so now it doesnt"
With the third, the DM says "The monster targets you with X, his roll is 20, that hits your AC, he deals 10 damage", and you say "I'm going to use Parry, and reduce the damage by 1d6".
With Shield, it is up to the DM whether or not he reveals the total of the roll or not. I generally think it is good practice to do so, but the DM is within his rights to ask for your ACs and keep track of the changes (this also creates more bookkeeping for the DM which can be a pain, just put your cards on the table).
It is likewise, up to the DM whether he tells you how much damage is done before Parry is applied. However, in general, I'd also prefer open communication here.