ok, so as has been said you don't want the bow to curve in a "sad" face but in a smily face, this concave curve is like a spring, so that when you tighten the hairs it makes tension on them and keeps them taut, and when you loosen you obviously let the spring go. just like any normal spring a bow can be overstretched and then it won't work as effectively. this is why you loosen the bow after play to keep it springy.
a good way to check of your bow is tight enough is to wind it in a little and then place it on the string, if you press and the wood touches the string very easily then its too loose, if it is really hard to push down on the bow and it doesn't move then it is to tight. you want to be able to move the bow up and down with your first finger of your bow hand easily so that the wood can get closer and further away from the strings, but not so that it touches easily.
ok thats enough of a lecture! sorry it went on!
i hope that helps but you'll get used to where is easiest to play for you.
hi. i've been playing the violin for 3 years. You will learn! you need to tighten it enough that a pencil stuck inside will easily fall out if you tilt it. Although this technique worked for me, asking a teacher is the best way to go