Let's try to make up a sort of short summary (sorry DoN, but i'm not talented as
@Malisteen for this sort of thing...
Before Legions of Nagash and 2nd edition, things were kinda like this:
Bringing back dead models in a unit: the banner of the unit was able to raise a certain number of dead models (depending on what kind of unit. for skeletons it was a d6, for guards a d3)
Summoning units to the table: you had to keep in reserve a certain number of points (example: in a 2000 pts army, you could field 1700 pts… 300 pts were kept away, to summon units during the game, with the appropriate spell)
Things were not limited to this, but this was more or less how it worked.
Now, things are changed.
AoS second edition: basically, to summon new units (or bring back destroyed ones) doesn't require to have a pool of reserve points. If you meet a certain number of conditions, you can do it for free.
Legion of Nagash.
When you play LoN, you can place 4 gravesites on the battlefield, that work in many ways:
1 - you can "keep away" a unit with the keyword "summonable", without placing it on the battlefield, and summon it n the battlefield when a hero is near a gravesite.
2 - You can raise dead models in a unit with the keyword "summonable", tnx to the abilities of heroes (that work on all the battlefield) and that number can increase if the unit is also near a gravesite.
3 - if a "summonable" unit is destroyed, you can bring it back in its entirety for free, if you have a hero near (9") a gravesite, fielding the unit within 9" from that gravesite, at more than 9" from enemy models.
If you play nighthaunts using their new battletome, then there are no gravesites, but heroes (and magic) can bring back dead models
If you play Flesh Eater Courts, also in that case heroes let you bring back dead models
NEXT TO COME: some useful basic trick….