"Given that" refers to something which is assumed or known to be the case. For example, given that you have asked this question, it is reasonable to expect that there will be people who answer it.
Given that 6 pears cost £1.80 the average price of those pears is 30 pence each.
Given that I was born in the 80's, I must be at least 29 years old.
I have answered this question with a few examples but, even given that, you may not quite grasp the meaning of "given that" properly, in which case feel free to ask follow-up questions.