What are Dex saves for?

Dex saves only cause movement/prone if specifically stated; you can handwave the rest away with flavor. For the fireball example, the DEX save is to throw up your arms and curl up to minimize fire exposure; it’s why you take half on a failed save and not none.

Does Shield Master give shield proficiency?

@JeremyECrawford @mikemearls Shield Master feat has no shield proficiency requirement.

Do you add anything to death saves?

Death saving throws do not use an Ability Score Modifier. Some newer players seem to think you add your Constitution modifier to them. I’ll admit, it makes sense to think that way. But, this isn’t the case. Death saves do not use your Constitution modifier.

Can you add shield bonus to dexterity save throw?

If you aren’t incapacitated, you can add your shield’s AC bonus to any Dexterity saving throw you make against a spell or other harmful effect that targets only you. So you do not gain the save bonus for AoE spells.

What is the second effect of shield Master?

The second effect of the 5e feat “Shield Master” reads (emphasis mine): If you aren’t incapacitated, you can add your shield’s AC bonus to any Dexterity saving throw you make against a spell or other harmful effect that targets only you. The wording here is ambiguous.

What kind of spells does shield master work against?

Shield Master does indeed only work against single-target spells. From the PHB chapter on spellcasting (page 204): A spell’s description tells you whether the spell targets creatures, objects, or a point of origin for an area of effect (described below).

What is the effect of cover against spells?

I don’t see rules of cover explicitly for 5e, against spells. 1/2 and 3/4 cover do +2 and +5 respectively against attacks and Dex saving throws, but what about spells? Does it only matter if the spell requires an attack roll and all others succeed automatically?