Electron affinity of an atom describes the quantity of energy that is released when a single electron is added to an atom. Electron affinity can be either positive or negative meaning that certain ions of an element can be more stable than the neutral element.
As the electron affinity is the energy released when an electron is added, it is positive when the atom is more stable after the electron was added. This means for most non-noble gases the electron affinity is positive.
The noble gases have negative electron affinities as they would need an additional shell for the additional electron.