According to the official white paper:

TRON is a world-leading blockchain-based decentralized protocol that aims to construct a worldwide free content entertainment system with the blockchain and distributed storage technology. The protocol allows each user to freely publish, store and own data, and in the decentralized autonomous form, decides the distribution, subscription, and push of contents and enables content creators by releasing, circulating and dealing with digital assets, thus forming a decentralized content entertainment ecosystem.
If you are confused about the excerpt above, here is a simple explanation:

In today’s modern world, almost all content creation on the Internet is forced through giant conglomerate companies such as Google, Youtube, Apple, Spotify, etc. If you are a creator and want to make money from your website, videos, music, blogs, etc; you must go through these giant companies, which serve as “middle men”. They provide the platform for your content, but also take a percentage of the profit your content generates through clicks, likes, views, streams, listens. They have full control over how much THEY want to pay YOU for the content YOU created. In addition, not only do these companies control how much you earn for your content, they also have the ability to censor and control what you create for your content.

This problem with censorship has become a HUGE issue within the Youtube community, sparked by Youtube demonetizing videos that do not fall in line with their “advertisement-friendly” guidelines.

In August 2016, YouTube introduced a new system to notify users of violations of the “advertiser-friendly content” rules, and allow them to appeal. Following its introduction, many prominent YouTube users began to accuse the site of engaging in de facto censorship, arbitrarily disabling monetization on videos discussing various topics such as skin care, politics, and LGBT history.

Solution? TRX and the TRON protocol!

As stated in the whitepaper, Tron’s goal is to create a worldwide free content entertainment ecosystem. They plan to accomplish this by cutting out the giant corporate “middle men” and putting all the profit earning potential back into the content creator’s hands. This means that when a user of the TRON protocol creates content, let’s say for example, a video, they can upload it directly to the TRON blockchain and get paid directly for how viral their video gets. Each upvote, like, clap, view that their video gets, they will receive direct compensation for, without the need of large companies like Youtube cutting into creator profits. The worldwide community on the TRON protocol will decide what it deems worthy of success, without the problem of censorship or corporate control looming over the heads of creators.

It is important to note, becoming the solution to the problem above is not TRON’s only goal.

Solving the problem of centralized entertainment content may be a “Micro” goal for TRON, but the “Macro” goal is much greater. TRON’s “Macro” goal is to completely reshape how we as a global society interact via the Internet and blockchain as a whole on a truly decentralized platform ecosystem.

One way to view TRX is to view it as the “exchange” currency on the TRON protocol blockchain.
There will be many companies built on the TRON blockchain that will use their own individual company cryptocoins, but TRX will be intertwined as the mediator between all of them.