"A box with stuff in it" is a term people use a lot to describe the way that many first-time designers end up creating their maps. This graphic from
Robert Yang (website is nsfw-ish) is particularly helpful:
(rehosted on imgur because the forums don't seem to like embedding twitter images)
Levels that play well often don't resemble the kinds of places we are used to. If you've grown up in a rural or suburban area, you're probably used to seeing buildings that are their own independent structures, with yards surrounding them. Look at a lot of early TF2 and CS:S maps and you'll find that the layouts more closely resemble dense urban settings, where any open space is surrounded on almost all sides by high walls. If you look at the "Intermediate" drawing, you can see this pretty clearly: Instead of one giant box with a lot of objects in it, they've gone with a series of smaller boxes that only have a few things in each.
Dividing your map into smaller chunks means that you have much greater control over sniper sightlines (because you have more walls), and you also give players more options. A large open field seems like the space where players have the most freedom, as they can move in any direction! However, when you create an incredibly large space with little cover, moving from one place to another doesn't mean as much. By providing more walls, you're creating more places where players can take advantage of the geometry around them for fighting enemies.
Dividing a map into smaller chunks also provides a lot of benefits in routing/map control, but those are more complex topics that are maybe better left for later.