Electron affinity refers to the amount of heat energy released when a mole of a gaseous atom combines with a mole of electrons. Greater the tendency to combine with electrons, higher is the electron affinity values.
The elements between Al to Cl are Al, Si, P, S and Cl having outershell electronic configurations as `3s^2 3p^1, 3s^2 3p^2, 3s^2 3p^3` , `3s^2 3p^4 ` and `3s^2 3p^5` . The values exhibit a gradual increase with one irregularity at P. This is because as we move from Al to Cl, one extra proton is added each time to the nucleus having same outer orbital dimensions. Thus, there is an increase in effective nuclear charge which results in extra attraction betwee the incoming electrons and the nucleus. This is reflected in the regular increase in electron affinity values.
However, for P (`3s^23p^3` ) the p-orbitals are exactly half-filled. Hence, enjoys extra stability on that count. Adition of an electron to that system robs it off this element of extra stability. That is the reason P shows an abnormally low electron affinity value.