What is Electron affinity?

Example 2.14 (Halogens and their neighbours)

The electron affinities of $\ce{F}$, $\ce{Cl}$, $\ce{Br}$ and $\ce{I}$ are 3.40, 3.61, 3.36 and $3.06\,\mathrm{eV}$, the largest of all elements; those of $\ce{O}$ and $\ce{S}$ are 1.44 and $2.02\,\mathrm{eV}$, of $\ce{C}$ $1.26\,\mathrm{eV}$, of $\ce{Na}$ $0.55\,\mathrm{eV}$. Atoms that complete a subshell by capturing an electron (the halogens) release much energy. Nitrogen, beryllium, magnesium and the noble gases form no stable gaseous anion: the extra electron would have to pair in a half-filled p subshell or enter a new subshell. Fluorine’s affinity is smaller than chlorine’s because the added electron is crowded into the small 2p shell.