1. Concerning altitude/flight level, the term means to remain at the altitude/flight level specified. The phrase "climb and" or "descend and" normally precedes "maintain" and the altitude assignment; e.g., "descend and maintain 5,000."
  2. Concerning other ATC instructions, the term is used in its literal sense; e.g., maintain VFR.

Source: Pilot Contoller Glossary (PCG)