Humans? Intelligent! Potted plants? Not so much. Intelligent means smart, or having the ability to process and understand information (including dictionary definitions).
Intelligent comes from a Latin word meaning "to understand, to gather," and intelligent creatures gather and make sense of information easily. Intelligent also refers to the ability to think. Intelligent objects, like speakers or software, seem to think in the way they react to change. If we’re looking for intelligent life on Mars, we want Martians who can reason. Intelligent also means showing good judgement, like an intelligent decision not to invite all those Martians to your birthday party.