The block time is an average time. It is a measure of how long it will take the hashing power of the network to find a solution to the block hash. The difficulty is calculated proportionally to the hashing power so that, on average, the block time is 10 minutes for Bitcoin. In reality, block times can vary. It is different for each Cryptocurrency.
Bitcoin Block Time - 10 Minutes
Ethereum Block Time - 20 sec
In 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.
A block is nothing short of a series of transactions compiled together. It reference to the previous block and the solution to a mathematical problem.
The bitcoin protocol produces a new block, on average, every 10 minutes. Miners are making trillions of guesses every second, trying to find the right nonce value that will make the block valid.
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.
I hope this helps !