CEFR stands for Common European Framework of Reference for Languages. It sets out a range of language aptitude levels that can be applied to all European languages when learned as a second or other language.
Courses and examinations in English as a foreign language are increasingly tied to CEFR levels, both in Europe and around the world. The six levels range from A1 (beginner) to C2 (fluent, near-native and able to use English in professional or literary contexts).
The Council of Europe provides a useful self-assessment framework for skills in reading, writing, speaking and listening. You can find it here.
Many titles in the Usborne Reading Programme have been assessed for their CEFR reading level. When you find a book you like, look for the CEFR level in the Book Information panel.