There are a number of ways to think about this, but one of them comes from "design thinking", which advocates processes like brainstorming in order to generate and sort ideas before starting a project. This is in contrast to what I sometimes think of as the "natural" or "naive" approach to starting a project, where you might spend time searching for "an idea", and when you've found "the idea", you begin executing. Turns out, methods of ideating reveal that a single person or a group of people can easily come up with thousands of good ideas in a matter of hours. After participating in these sorts of activities, I find it impossible to assign much value to an idea. The actual value, or lack thereof, only reveals itself much later and involves many more factors than the initial concept.

Here's another one: what is an idea? Typically it is a sentence expressing a business proposition, or the essential functioning of a technology. But is it the same "idea" no matter who utters it? I think it's pretty clear that an idea is actually short-hand for a 'space of experience' held by the person saying it. In this sense, it's actually not possible to "steal" an idea, because ideas are linked to the people who have them, and because were someone to try, they would find that they can steal the sentence that refers to the actual idea, but lack the perspective to unlock the true meaning.

Either way, this is a meme in the startup industry, and very true in my experience. Do not fixate on ideas: nobody can steal them and/or they are not important in the first place.