What is Stray Capacitance and how it can be Avoided
Whenever two pieces of metal are near each other, there is capacitance between them. If there is a voltage difference between them, there will be some equal and opposite charges accumulated on the two "electrodes". Conversely, in order to establish a voltage difference between them you must supply enough charge produce that voltage.

So if your signal in your rf circuit travels along a metallic conductor (like a wire or PCB trace) and that conductor is near some other conductor (like pins of a chip, or the lid of your enclosure or mounting screws or whatever), there will be capacitance between those two objects. If that isn't capacitance that the designer intended to exist, then we call it "stray capacitance".

To be honest, usually in rf we design the signal paths to be reasonably well shielded from sources of stray capacitance, and so this isn't usually an issue (but I've never tried to do rf on a breadboard). It's more in precision analog design that I've seen problems with stray capacitance.