The concept of "shower thoughts" has become pretty synonymous with the subreddit of the same name.
<aside> ☝ Side-note on a side-note: reddit was an organic idea for a startup by a UVA student.
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And while that subreddit focuses mostly on pithy musings or unique perspectives on mundane things, it hits on something pretty interesting that Paul Graham notes on his blog:
The shower is a sort of meditative space where your mind is able to wander, but not completely unguided, so you arrive at great ideas that you wouldn't have found if you were fully focused on finding them$^1$
And this has pretty powerful implications for you as an entrepreneur (or as an inventor, or poet, or songwriter, or anything else) because the things that you allow your mind to latently think about are going to end up producing fresh ideas. This is valuable regardless of the stage of the entrepreneurial process you're in, but it's especially useful during ideation. If you want to produce good insights around your entrepreneurial idea (or your inventions, poems, etc.) you want to make sure that other things (e.g. bills, a text you just received, raising money) aren't getting in the way. Therefore, you need to come into the shower with whatever problem you're solving at the top of your mind.
It can be difficult, as an entrepreneur in general, to focus on just one thing. Founders that I've heard from identified an impulse to keep jumping around, building new things, experimenting on new projects, and generally not being too tied to a single idea. This is an excellent way to think in the very early stages of ideation for your projects, but once things start to take off, staying loyal to too many ideas will wind up spreading you very thin. The solution: commit fully to the idea that is the furthest along in the process or that you believe in the most, let the others be picked up by others, or move them to your back burner$^2$.