A front-end framework is a framework specifically for the front-end. Yes I know, that has helped you a lot. Front-end specifies the side of a program that the user gets to see, when we think of a webshop, the shop that you the customer will interact with is the frontend. the back-end is the part that does the logic. That is a rough explenation, there is more to it than that, but this is a simple example.
Now when it comes t front-end we are mostly thinking about html, css and javascript. So when we take this combination, and make it simpler to use, make it easier to create your designs. That is a framework. Again, in rough lines. A framework lets you build more to make your website the product you want it to be, rather than programming the system that you can later use to easily make your product.
Now this is a rough explanation, there is a lot more that goes into it.
There are a lot of front end frameworks, but I am not just going to list front-end frameworks only, here is a list of javascript frameworks that can be either front-end, back-end or both.