The block time of a block is the amount of time that it took to solve that block. It is calculated by subtracting the time of the block in question from the time of the block before it.
The aim is to have blocks mined at an average of a block every 2 minutes. The actual time may vary widely, with some blocks being solved in seconds while others taking several minutes to complete. The difficulty is adjusted up and down depeninding of the amount of hashing power currently mining Electroneum worldwide, to ensure that an average rate of 1 block every 2 minutes is mined.