A more formal definition of a meta object can be given as a directing structure which can be seen as the source of a static field. The field can be either positive or negative and hence the field generated by neighboring directing structures can attract or repel.
The implicit surface is defined as the surface where the 3D field generated by all the directing structures assume a given value. For example a meta ball, whose directing structure is a point, generates an isotropic (i.e. identical in all directions) field around it and the surfaces at constant field value are spheres centered at the directing point.
Meta objects are nothing more than mathematical formula that perform logical operations on one another (AND, OR), and that can be added and subtracted from each other. This method is also called Constructive Solid Geometry (CSG). Because of its mathematical nature, CSG uses little memory, but requires lots of processing power to compute.
The Wikipedia page about metaballs.
Blender has five types of metas, each determined by its underlying (or directing) structure.
In Edit Mode, you can change this structure, either using the relevant buttons in the Active Element panel, or the selector in the Transform panel in the Sidebar region. Depending on the structure, you might have additional parameters, located in both Transform panel and Active Element panel.
- Ball (point, zero-dimensional structure)
This is the simplest meta, without any additional setting. As it is just a point, it generates an isotropic field, yielding a spherical surface (this is why it is called Meta Ball or Ball in Blender).
- Capsule (straight line, one-dimensional structure)
This is a meta which surface is generated by the field produced by a straight line of a given length. This gives a cylindrical surface, with rounded closed ends.
- Size X
The length of the line (and hence, of the capsule).
- Plane (rectangular plane, two-dimensional structure)
This is a meta which surface is generated by the field produced by a rectangular plane. This gives a parallelepipedal surface, with a fixed thickness, and rounded borders.
- Size X/Y
The length and width of the rectangle.
- Ellipsoid (ellipsoidal volume, three-dimensional structure)
This is a meta which surface is generated by the field produced by an ellipsoidal volume. This gives an ellipsoidal surface.
- Size X/Y/Z
The length, width and height of the ellipsoid.
- Cube (parallelepipedal volume, three-dimensional structure)
This is a meta which surface is generated by the field produced by a parallelepipedal volume. This gives a parallelepipedal surface, with rounded edges.
- Size X/Y/Z
The length, width and height of the parallelepiped.