To visualize means to create an image or picture in your mind's eye. It's like closing your eyes and picturing a scene or object.
When people say they're going to "visualize" something, they usually mean that they want to imagine it clearly before taking action. This can be helpful for tasks like planning a project, remembering a face, or even practicing a sport. For example, if you're an athlete preparing for a competition, you might try to visualize yourself performing well and making good shots. Similarly, designers use visualization techniques when creating new products or buildings - they'll often sketch out what it will look like before starting construction.
There's another meaning of "visualize" that comes from the world of photography and art. In this sense, "to visualize" means to translate an idea into a visual form, like a picture or video. For instance, a graphic designer might be asked to visualize a company's new brand identity - they'd create images and graphics to represent the brand.
To envisage, or form a mental picture (of something).
