The ISTQB Glossary of Testing Terms defines a test tool as:
A software product that supports one or more test activities, such as planning and control, specification, building initial files and data, test execution and test analysis.
Therefore a test tool can be thought of as a piece of software that is used to make the testing process more effective or efficient. In other words, anything that makes testing easier, quicker, more accurate, etc.
This book will focus on those test tools that are listed in the syllabus. These are, in general, the test tools that are most commonly used in the testing process and designed primarily for the testing process.