In this sentence:

"Tardé pocas horas en hacerlo."

(It didn't take long to do it. [lit: I took few hours to do it])

The speaker is saying he/she took few hours (as in not too many hours, so it has a positive meaning)

If we changed it to:

"Tardé unas pocas horas en hacerlo."

This would mean "It took me a few hours to do it" (Here it is implying that it took long, so it is more of a negative meaning.)

If you have a look at this other sentence taken from the lesson:

"Quedan pocas cervezas en la nevera."

There aren't many beers left in the fridge. [lit: there are few beers left]

and now see how it turns into something different if you use "unas pocas":

"Quedan unas pocas cervezas en la nevera."

There are a few beers left in the fridge. 

The second option with "un poco de", "unos pocos..." "unas pocas" is always more literal in English, "a bit of", "a few..." 

Hope this clarified it for you.

Saludos

Inma

Hola Alyssa, 

There is a block in the lesson saying that when we use un poco de + a noun singular, the word "poco" is invariable, so, even with feminine nouns we still say un poco de... nor una poca de...

Saludos

Inma