What is electron affinity?
Electron affinity is defined as the enthalpy change for the addition of 1 mole of electrons to 1 mole of atoms in the gaseous state.
E.g. for chlorine:
Electron affinity is basically the amount of energy required for an atom to be able to "snatch" and electron from another atom. In other words, how much energy is an atom willing to spend to get another electron?
Electron affinity increases as the number of valence electrons in the outer shell increases (doesn't rise beyond 8). This makes sense, because we know that atoms with many valence electrons in their outer shell want that final, extra electron to complete their octet, and be happy eternally. Hence, they'd be willing to invest a lot more energy in getting that.
Hope that helped :)