Electron affinity is the energy released when an electron is added to an atom. The trends are upward and towards the right of the periodic table.
Think about it like how badly an atom wants an electron. The halogens (the 7th column) want an electron SO BAD because they just want to be a noble gas, and it only takes 1 electron to be added. So they will have then HIGHEST electron affinity, and the most energy will be released.