What is a Barlow lens? I am looking to buy a Celestron 21023 FirstScope 76 Telescope, which is quite inexpensive, and it has good reviews on Amazon.  One thing that many reviewers have said is that the two lenses that it comes with are of low quality, and the suggestion is to toss them and buy a Barlow lens, and perhaps some other more quality lens.

I don't know what a Barlow lens is.  

Is this a brand name?  The name of a type of lens?  What distinguishes it from a non-Barlow lens?
I have been an amateur astronomer for over 30 years so I can give you some practical advice on this one.

A Barlow lens is an attachment which fits into the eyepiece holder. It's main purpose is to give increased magnification with your existing eyepieces. They are usually marked as 2x or 3x which indicates how many times they will multiply the magnifying power of an eyepiece. They can be useful for large telescopes which can comfortably handle high powers but for a smaller scope they can be of limited use. 

The two problems you would encounter if you used one are, first, that the more you magnify an image the fainter it gets, with a smallish diameter scope like yours this means that you will find it very difficult to see faint objects. The other one is the fact that locating and tracking objects as they move (due to the rotation of the earth) becomes much harder as you increase magnification, the field of view of the eyepiece gets much smaller as you increase the power.

I would personally suggest that you get used to using your new scope for a while as it is, then, see whether you might need one in the future. If your main area of interest turns out to be the moon and planets then a Barlow might be useful at some point, but if you enjoy wide field views of nebulae and star clusters then that is less likely.
Hope this is of some help.