Front end is a term used to describe an application or website. It denotes the way to access the data, which is often referred to as the back end.
This is especially prevalent with blockchain technology. The database on a blockchain is available to any application. Hence, they all share the back end.
To illustrate, Twitter has both a front and back end. The user interface that everyone sees is the front end. With Twitter, the corporation controls all the back end data, making it available only to its UI.
It is how all centralized applications work, especially with social media and finance. Bank information is theirs, not shared with anyone else. Their front ends are the only ones to access it.
With blockchain, multiple front ends can not only read the same data but also offer identical services. Nobody has a monopoly simply due to the openness of the database. One benefit is that accounts are available to each along with features such as follow lists (for social media purposes). This eliminates the idea of a siloed system which is common for Web 2.0.
This is a large part of the concept of Web 3.0.