Adverbs of probability tell us the likelihood of something happening.
If you imagine playing dice, what's the likelihood (probability) of rolling a six?
You know it's possible, but it's not certain.
The only certainty is that you'll throw something between one and six. However, your less likely to throw two sixes.
Adverbs of probability include; certainly, definitely, doubtless, maybe, perhaps, possibly, probably etc.
For example:
We will win the game. = We will certainly win the game. - In this sentence certainly shows us the probability.