A contact hour is an hour actually spent in the classroom teaching and working with students. (It is also sometimes ironically referred to as an hour at the chalkface.)
Many schools will employ TEFL teachers for, say, between 20 – 30 contact hours per week (this is moderately high; some will be less). Thirty hours is already quite a lot and more than this will mean you are spending too much time in the classroom. This is because you need to remember that on top of contact hours there will preparation time where you will be planning your lessons, marking work and meeting with teachers/admin staff at the school. These extra hours can easily take your total work week over 40 hours.
Note that when you read through the teaching contract you should check whether the weekly hours they mention are just contact hours or whether they include preparation time, etc. You should also consider traveling time; you may have spend time traveling either to and from work or possibly between schools. In addition there may be split shifts where you work a couple of hours or so in the morning then have 3 or 4 hours off and then work a few more hours in the evening. This can result in spending more hours between lessons than actually teaching!