It's defined in the equation
where A is the activity and N the number of nuclei.
This is also written as
It's the constant of proportionality in the equation relating the activity of a sample to the number of active nuclei present in it.
The physical significance of the constant is one of probability.
It signifies the probability of an active nucleus decaying in a interval of one second.
Or, the fraction of nuclei that decay in a sample in unit time.