The convection zone of a star (e.g., the Sun) is a region within its interior where energy is transferred outward through convection, energy moved outward through the motion of hot material. This occurs when some layer within the star expands due to high temperature, and its density falls below that above it (outward of it), an instability, and the layer's material is drawn away from the star's center of gravity through buoyancy. The condition causing this is a layer with opacity sufficiently high that radiative transfer is too inefficient to move the energy and relieve the heat within the lower layers. In general among main sequence stars, the smallest, such as red dwarves have convection zones through much of the star, mid-sized stars, such as the Sun, have a convection zone in the outer portion, and massive stars such as A-type star and above have a convection zone within their stellar core, in some cases extending considerably outward. Pre-main-sequence stars and post-main-sequence stars also pass through phases with considerable convection.
Radiation zones are other regions of the star's interior volume, in which radiative transfer is dominant. The term radiative-convective boundary (RCB) indicates the border between the zones.
The terminology is used for planets as well, particularly gas giants such as Jupiter and Saturn. I've seen the term RCB used more often for planets and the term convection zone more often for the Sun.