I hear it a lot in engine design , I think that belongs to some phase about we can change the engine parameters according to requirements but I am not sure about it.
A rubber engine is not to be confused with a rubber motor (a rubber band which is twisted in order to store energy which can be released when the rubber band untwists. This type of engine is good for short energy bursts in small model airplanes).
A rubber engine is an engine deck (tables of engine data) which can be scaled according to your needs. In a way, you stretch one parameter of the data like a rubber band, and all other parameters change accordingly. In most cases, you adjust thrust to your needs and the scaled engine deck will produce mass, size and fuel consumption for the most likely actual engine with the given thrust. This is very helpful in the initial stages of engine design when you search for the smallest possible airframe to fulfil given mission requirements.
I intended to make this a comment on Peter Kämpf's answer, but don't have enough reputation.
In my engineering education, a "rubber engine" referred to an engine with variable parameters. A "fixed engine" referred to a specific engine with well-defined parameters, for example "Lycoming O-235-F1." In general, you use a rubber engine in aircraft design when you know generally what type of aircraft you're working on, e.g. a single engine piston-propeller or a multi-engine turboprop, and you just need values that are "good enough" without restricting your design to a specific model. Once you have an idea of what the aircraft's parameters are going to be, you can select a specific engine model and modify the rest of the design to work with the newly-restricted engine.
It's not as restrictive of a definition as a type of data table; it can be as simple as having single numbers for power, fuel consumption, weight, etc.