So I peaced out of DnD with the introduction of 3rd ed. I understand it did well, then 4th came out and bombed and now there is 5th ed. What has changed?
I can't tell if you are asking what has changed between AD&D and 5e or if you are asking what changed between 3e, 4e, and 5e.
For the former:
stats are standardized bonuses, +1 every two points above 10 instead of AD&D reverse bell curve.
No class or race or level or alignment restrictions.
Some new core classes and races.
AC starts at 10 and goes up, instead of THAC0 you have an attack bonus that goes up as you level.
A skill system where you add stat bonus and if trained in skill a level based bonus (this is how saves are done as well). Always trying to roll high on a d20 add bonuses against target number.
Saves are based on stats (dex save) instead of category.
Multiclassing is take individual levels in different classes when you level up and not split xp between multiple classes.
All classes are designed to be balanced in combat at every level and use the same xp chart.
More class powers for everybody.
Magic slots are more flexible and easier to cast in combat, but many spells require concentration so less stacking spells and none go up by caster level.
Spell save target is based on caster stat bonus and not an unchanging base.
Initiative is one at a time, only roll once at beginning of combat.
Fully heal up and regain powers at end of day.
Regain less powerful things and can nonmagically heal some after short rest during day.
At 0 hp you go unconscious and start making death saves instead of going to negative hp.
Alignment is not mechanically important.
Monsters have full stats including con bonus to hp, many have a ton more HD.
Instead of energy drain monsters reduce max hp (so affects healing up only) until long rest at end of day removes that condition.
Many powerful magic items require attunement to work, can only have three attuned items.
Core rules are free online in basic set and SRD.
Those are the big ones I'd say.