What makes probabilities independent?
Probabilities are 'independent' when they do not need any previous condition to occur.
It may be easier to think conceptually of a dependent probability. For example if the probability of going on an outing depends on the probability of rainfall on a particular day, it is dependent. The probability of the event would require combination of BOTH the rain probability and the outing probability.
The rainfall, on the other hand, is an example of an independent probability.