When you provide a specific example to illustrate an idea, you instantiate it. You say you believe in unicorns, but so far you haven't been able to instantiate that belief.
Philosophers often use this term, which can be most simply understood as "represent by an instance." A multi-millionaire may choose to instantiate their wealth by driving an extremely expensive car and wearing $500 shoes. A photojournalist might attempt to instantiate the dangers of a particular intersection by taking photos of car accidents that occur there. Whenever you clarify or explain a concept with a real-world example of it, you instantiate that idea.