Redshfit Automation adds a few weapons and other fun things, such as a custom boss, music for the vanilla bosses, and more.
Here are some of the major additions:
Redshift has been extensively playtested before release, but it is built for a certain kind of game. It is great for messing around with your friends. It is not great if you want an extremely well-balanced, difficult survival experience.
A mod like JEI is highly recommended to view the crafting recipes.
The Redshift Core, which is used to craft all of Redshift's tools and such, is obtained by killing the Automaton. The Automaton is a custom boss made by placing a redstone block on top of a smooth stone block.
The Automaton will usually attack by spamming arrows. (Its boss track is called "Wall of Arrows" for a reason.) Prepare accordingly. It can also call down glider raids and summon Operatives, dangerous semitransparent mobs that carry Electron Swords.
It will also drop lots of useful items other than Redshift Cores. It can Redshift items, materials used to make them, and boatloads of iron.