You have 4 fruits: apple, banana, orange, strawberry.
Now you put the first one into the mixer => apple, 3 fruits left.
Now you put the second one into the mixer => apple + banana, together applebanana, 2 fruits left.
Now you put the third one into the mixer => applebanana + orange, together applebananaorange, 1 fruit left.
This goes until all fruits are gone and we have applebananaorangestrawberry.
In my example, the reduce function would be (a,b) => a + b, because I add the fruits together. a is the accumulation of the fruits, e.g. applebanana, b is the next value, e.g. orange.
The next time, a would be. applebananaorange, b the next value, strawberry.