From the book's introduction:

This edition of RuneQuest is a heavily edited and pared-down version of the full rulebook. Although the core of the system is intact, this Essentials Edition is intended to be a sampler for the main rules and, hopefully, encourage players and Games Masters to invest in the full game. For example, we have only included two of the five magic systems; we have also left out the extensive chapter concerning cults and brotherhoods, and have provided only a fraction of the creatures. Nevertheless, there is enough here to get started and, indeed, enjoy many hours of adventure. At some stage though, you will, we hope, want more, and be curious and eager enough to buy the RuneQuest 6th Edition core rules so that you can take advantage of the complete system in all its glory.

From the publisher's web site:

This special version of RuneQuest 6th Edition is designed to introduce players and Games Masters to the essentials of the game. In this book you will find the core of the RuneQuest engine, in a stripped-down format, but still complete enough to create characters and begin adventuring with one of roleplaying’s most enduring and respected systems.

The intention is that this book will give you a taste of the mechanics and encourage you to move onto the full edition of the rules. The full rulebook contains much, much more: more game options, more magic, more monsters more cults.