When you have a lot of objects, it is referred to as "crowded space," as in a living room cluttered with too much furniture. When you have a few objects it is referred to as "empty space," as in a newly moved into residence where there may be little to no furniture and a lot of empty room.
Positive space is where shapes and forms exist. Negative space is the empty space around them. For instance a large bedroom that contains an oversized bed that is overly decorated with fine bedding and accessories could be complemented with walls that are bare or that have minimal decoration as the empty space could balance out the extravagance of the bed. Negative and positive space work in tandem to balance each other.
Space may have two dimensions --- length and width --- such as a floor or three dimensions --- length, height and width --- such as a table. Space is also the area around, between and within objects. When it comes to positive space it can have a similar or same meaning as shape.
The "3 Fs" of design are "form follows function." Form refers to what something looks like and function refers to how it works. Space is one of the basic units of design along with line, color, shape, form and value. The spacing of objects can have a profound impact on design --- form and function. When you enter a well-designed room and if an object is out of place it is instantly noticeable. If your aim is to achieve symmetry in a room, then objects need to be equally spaced on either side. This is why beds are often complemented with two bedside tables on either side.