What is the area of a 5 foot square in D&D?

Could someone please clarify for me the area measurement of a 5 foot square in the context of Dungeons & Dragons? I'm trying to grasp the concept of how much space it represents and how it might impact gameplay decisions, such as positioning and movement. I'm not entirely sure if it's just a simple square with sides measuring 5 feet, or if there's more to it than that. Any insights or explanations would be greatly appreciated.