To put it simply it means if you want to get revenge for something somebody has done to you, don't do so in the heat of the moment, wait until you calm down, bide your time and then take your revenge, in a cold and calculated way. Sounds awful doesn't it!
agreein with horselady - the point is that you will exact better revenge if you wait and think it through instead of rushin into something in the heat of the moment. It's a brilliant and very apt saying.
Also, if you wait until you cool down you may decide against trying to exact revenge at all, which is frequently the wisest course. There was a Sicilian saying which ran 'when you seek blood revenge, better dig TWO graves'