We use the Common European Framework of Reference for Languages (CEFR) to indicate the levels of our resources. Many lesson plans and activities are flexible and suitable for students learning at different levels.
If you are not familiar with the CEFR, this is an approximate explanation using terms common in many staffrooms:
A1: Beginner
A2: Elementary
B1: Intermediate
B2: Upper-intermediate
C1: Advanced
C2: Proficiency