The answer is TRUE
There are only two possible outcomes
- Tomorrow it rains.
- Tomorrow it does not rain.
Thus the probability of one outcome dictates the probability of the other.
For example, imagine that
P(It will rain tomorrow) = 1 (This means that you are 100% sure it will rain)
Then you can assure that
P(It will rain tomorrow) = 1 - P(It will not rain tomorrow)
P(It will not rain tomorrow) = 0