If someone asks you what a shape is, you'll likely be able to name quite a few of them. But "shape" has a specific meaning, too—it's not just a name for circles, squares, and triangles.
A shape is the form of an object—not how much room it takes up or where it is physically, but the actual form it takes. A circle isn't defined by how much room it takes up or where you see it, but rather the actual round form that it takes.
A shape can be any size and appear anywhere; they're not constrained by anything because they don't actually take up any room. It's kind of hard to wrap your mind around, but don't think of them as being physical objects—a shape can be three-dimensional and take up physical room, such as a pyramid-shaped bookend or a cylinder can of oatmeal, or it can be two-dimensional and take up no physical room, such as a triangle drawn on a piece of paper.
The fact that it has a form is what differentiates a shape from a point or a line.
A point is just a position; it has no size, no width, no length, no dimension whatsoever.
A line, on the other hand, is one-dimensional. It extends infinitely in either direction and has no thickness. It's not a shape because it has no form.
Though we may represent points or lines as shapes because we need to actually see them, they don't actually have any form. That's what differentiates a shape from the other geometric figures—it's two- or three-dimensional, because it has a form.