This is a complete system to run fantasy and science-fantasy RPGs including system rules, characters, adventure creation and worldbuilding. It is generally compatible with older editions of D&D including B/X so there is a large pool of adventures to draw from. This version is totally and the value is astronomical.
The mechanics are simple and clear and should be familiar to anyone who has played D&D or other d20 games. Skills are rolled using 2d6 to get a slightly more predictable distribution than combat. Many of the systems that have atrophied away in more recent editions of D&D have been updated and really shine here: Encumbrance is a slot-based system that present players with good choices about what to carry and which items to prioritize for accessibility. In addition to hit points there is a stat called system strain that represents longer term wear and tear. System strain increases after receiving healing, through thirst, exposure or starvation, or due to illness or poisons. It connects a number of systems that make exploration and resource management meaningful without being unduly complex.
Character creation is a joy in this system. Players can choose to be a warrior, expert, mage or combination of two of those classes. Each class excels in its prime domain (combat, skills or magic) but none of the classes feel useless. There are a range of backgrounds and foci (like feats) that present a wide array of options for customization. If the low number of classes has you concerned there is nothing to fear. There are a large number of partial mage classes that can be combined to get a different flavor of character including the healer, necromancer, high mage, or elementalist classes.
The book contains extensive GM advice for running a sandbox style campaign and for creating content to suppor that play style. There are a tables to help create maps, kingdoms, governments, adventure hooks, treasure. There is great advice for creating different kinds of encounters (combat, exploration, social, mysteries) and for buidling character reputation and enabling faction play. There is information on creating monsters and a good number of templates but it does not rise to level of a complete monster manual.
This should be the default game for most people who are currently playing 5E and a great choice for anyone interested in making their own RPG content.
I have a complete review here if you are interested in the system in further depth: