"Better" is a comparative, i.e. it is a relationship between two things. "Best" is a superlative, i.e. it states the position of this one thing compared to all the other things under discussion.
If I have three choices, A, B, and C, all the following statements could be true: A is better than B. B is better than C. A is better than C. A is the best. If one thing is better than another, others could be better still. But if one thing is the best, than nothing can be better.
Note: The above assumes that "better" is a rating on some sort of scale. Of course A might be better than B in some contexts but not as good in others, for example an orange is better than a flashlight if I'm hungry but the flashlight is better if I'm stumbling in the dark. And one can imagine relationships like rock/paper/scissors where there is no best, just pairs of non-transitive relationships.