devtrackers.gg
At the very least, watch the intro of the first TF1. https://www.youtube.com/watch?v=OwcIdEhgVz4
The conflict of the Titanfall games is the Frontier War.
Hundreds of years ago, humanity discovered a way to fold space and time which greatly improved space travel. The main systems in which humanity inhabit are called the Core Systems, but people have also discovered a very faraway region of space full of habitable and terraformable planets called the Frontier. (Apex Legends takes place in the Outlands which is a section of the Frontier that was untouched by the Frontier War.)
The Interstellar Manufacturing Corporation (IMC) gave people land and money to settle in the Frontier. However, interest eventually waned and conflict broke out in the Core Systems, so the IMC left the Frontier; the people who settled in the Frontier stayed.
Generations later, the IMC came back to the Frontier and found that the Frontier worlds are prospering. The IMC wanted the Frontier's resources that they desperately need because the Core Worlds are running dry, and they believe it is their right to take it; legally speaking, that's probably true. The Frontier Worlds didn't like that since they've been living secluded from the Core Worlds and the IMC is taking their resources.
After negotiation attempts failed, the Frontier Worlds banded together to form the Frontier Militia to fight the IMC. The Frontier War is between the IMC of the Core Systems and the Militia of the Frontier. It ended with the IMC retreating from the Frontier back to the Core Systems. Unfortunately, the war ravaged most of the Frontier so the people moved to the Outlands where Apex Legends takes place and the Mercenary Syndicate, or just the Syndicate for short, took over.
If you're confused about factions, the IMC was originally Hammond Engineering created by Dr. Hammond. Through mergers, acquisitions, and rebrandings, it became the IMC. Hammond Robotics is one of the IMC's subsidiaries. Hammond Industries is another. There are many subsidiaries. (Currently, in Apex Legends, Hammond Robotics have left the IMC which is why they're back in the Outlands. The whole point of the Frontier War is that the IMC's gone so it wouldn't make sense for Hammond Robotics to be here if it's still IMC. Hammond Robotics is currently working with the Syndicate in the Outlands.)