Hey Rachada,
No need to be confused, Speaking Part Three on IELTS test sometimes has difficult questions and sometimes has topics which you may not know about.
It is a good idea to
try first to ask the examiner if they could rephrase the question or explain that you are not sure of the meaning of one of the words that you don't understand. However, if the examiner cannot change the questions and you still don't understand it. It would be best if you said that you are not sure about that exact topic, but you do understand something about a related topic ( must be something related to the topic ). This way, you are able to speak something.
It is always better to speak something, rather than nothing. For example, if the examiner asks you about eco-tourism, and you don't know what it means, ask for an explanation, and if you don't get one, then say something about tourism, as this would be a related topic. The point I made about telling the examiner that you do not have any ideas about that topic, is a good idea if you really have no ideas whatsoever 
. It is better to speak something rather than nothing, but it is also better to accept that you don't know anything about the topic rather than speak just for the sake of speaking and therefore speaking an answer which is completely unrelated to the question being asked.
These are just different strategies that a student can use during the Speaking test itself. In an ideal world, you will understand everything, answer all the questions really well and get a fantastic score!
Have fun,
Paul
