The CEFR is the Common European Framework of Reference for Languages.
It is a framework we can use to assess a language learner’s capabilities in a particular language (not only English). It is a table of descriptors relating to specific capabilities in different aspects of language. Teachers (and students) use these to allocate learners into appropriate levels, such as Elementary or Advanced.
The CEFR is divided into three types of user (Basic, Independent, and Proficient) and six levels of learner (A1, A2, B1, B2, C1, C2).
For example, an A1 user:
Can understand and use familiar everyday expressions and very basic phrases aimed at the satisfaction of needs of a concrete type. Can introduce him/herself and others and can ask and answer questions about personal details such as where he/she lives, people he/she knows and things he/she has. Can interact in a simple way provided the other person talks slowly and clearly and is prepared to help. 1
On the other end of the spectrum, a C2 user:
Can understand with ease virtually everything heard or read. Can summarise information from different spoken and written sources, reconstructing arguments and accounts in a coherent presentation. Can express him/herself spontaneously, very fluently and precisely, differentiating finer shades of meaning even in more complex situations. 1
In the EFL classroom, we often translate these CEFR levels into different labels, so that they can be understood more easily. For example:
A1: Beginner
A2: Elementary
B1: Intermediate
B2: Upper Intermediate
C1: Advanced
C2: Proficiency
Source: 1Council of Europe