Algorithm Running Time
Basically, the complexity of an algorithm gives the relation of the change in the running time with respect to the change in the input size. You can think it as,

$log_2(1M)$ -> 20 sec

$log_2(2M)$ -> ? sec

$log_2(2M) = log_2(2 *1M) = log_2(2) + log_2(1M) = 1 + 20 = 21 sec$