Course levels in EF Hello relate to CEFR (Common European Framework of Reference for Languages) levels, which is the standard structure for ranking language ability. Here is a guideline for how the EF Hello difficulty levels are matched to the CEFR levels:
Foundation = A1 CEFR
Beginner = A2 CEFR
Intermediate = B1 - B2 CEFR
Advanced = C1 - C2 CEFR
For more information about CEFR levels, please refer to https://www.cambridgeenglish.org/exams-and-tests/cefr/