n a more simple words, Block time is the time which the networks takes to generate one extra block or the time taken from one Block to another.No one can say exactly how much Block Time will it take to mine another. The 10 minute window is just an average. There is a generation calculator that will tell you how long it might take to mine a block.
Someone with good statics knowlege could calculate how average the block time is really. So normally it should be a Gauss normal distribution with the maximum at 10 mins. But it is not, cause e.g. miners are activated or deactivated and difficulty can not react. Look how many blocks are found exactly 10 mins after the last one or even 11 or 9 min.