The phrase "to enter the brain" means that information or thoughts are being processed and understood by the brain. It suggests that something is being comprehended or thought about by someone. Full definition
The blood - brain barrier is a highly selective barrier that prevents unwanted molecules and cells from entering the brain from the bloodstream. (sciencedaily.com)
With this minute size, they can enter the brain through the nose, lungs, and gastrointestinal tract. (techtimes.com)
«I wanted to understand how sensory information enters the brain and gives rise to thought,» he says. (discovermagazine.com)