One of the levels in the Common European Framework of Reference for Languages (CEFR). The sequence, from beginner to most advanced, is A1, A2, B1, B2, C1, C2.
Someone at the B1 level:
Can understand the main points of clear standard input on familiar matters regularly encountered in work, school, leisure, etc.
Can deal with most situations likely to arise whilst travelling.
Can produce simple connected text on topics which are familiar or of personal interest.
Can describe experiences and events, dreams, hopes and ambitions and briefly give reasons and explanations for opinions and plans.