Few
The word "few" emphasis on the lack of something.For example -
There are few chocolates left in the jar.
The above sentence tells us that we should be careful and not to eat chocolates too quickly because they are almost finished.
A Few
"A few" emphasis that something still remains.
For example -
We have a few minutes left in the class. Do you have any questions?
The above sentence tell that we still have some time left so we can use it wisely.