For people new to tabletop RPGs, this is the equivalent of a DLC expansion. It's new content, new rules, new classes, and so forth to augment your 5e game.
Notable contents include:
Racial stat bonuses can be moved around at will (i.e you can change a Half Elf's +2 Charisma to a +2 Strength)
Races with negative stat bonuses no longer have negative stat bonuses
A new "custom lineage" race exists, which allows to pick any race, and replace their features with a +2 to any stat of your choice, a feat, and darkvision.
These modify class features. Unfortunately, many of them are somewhat controversial in the community because people do not believe that they fixed many of the classes that are considered to have poor design, notably rangers and sorcerers.
For the spellcasters, spell versatility (a feature which allows you to change spells you know on a long rest) was not implemented, disappointing many
A few subclasses from other books are reprinted so you don't have to buy them (example: Eloquence Bard, from Mythical Odyssey of Theros), and a few are new, like Order Cleric, Wildfire Druid, and so forth.
In particular, the Clockwork Soul Sorcerer is one piece of good news for Sorcerer players.