It measn the same as "object" in other object-oriented languages, eg Eiffel, C#, Simula. It measn a computerised representation of "something" which has data (attributes, in Java called fields) and behaviour (it does something, which in Java is expressed through its methods).
What colour is a car?
"car" means a "class." "A car" is an "object." The "car" class can accommodate colour, speed, direction, fuel consumption, engine type, etc, etc, etc. "A car" might be a RED FORD, travelling SOUTH at 45 mph, using 1 gallon fuel for every 40 miles, with a Diesel engine.