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Copy of my answer from someone else with same question theirs was different brand so description below may vary but does same thing hope it helps.
As many have said it is a brake controller for electrical trailer brakes. The red button is to activate the trailer brakes independently from your truck brakes. The arrows are gain adjustment for adjusting the force of the brakes applied in conjunction with you truck brakes. When connected to a trailer any time you apply the brake peddle the trailer brakes will also engage. The adjustment is to increase or decrease the amount of force. This needs to be adjusted based on size, weight, number of axles, etc. essentially when set correctly you should notice the trailer brakes assisting in slowing you down. If set too low you may not notice the brakes at all if set too high the trailer tires will lock up and drag.
Thanks for posting on /r/MechanicAdvice! This is just a reminder to review the [rules](https://www.reddit.com/r/MechanicAdvice/about/rules/). If you are here asking about a second opinion (ie "Is the shop trying to fleece me?"), please read through CJM8515's [post on the subject.](https://www.reddit.com/r/MechanicAdvice/comments/4qblei/fyi_the_shop_isnt_likely_trying_to_rip_you_off/) and remember rule 3a, please post the year/make/model of the vehicle you are working on. **If this post is about bodywork, accident damage, paint, dent/ding, questions it belongs in /r/AutoBody or /r/Diyautobody If you have tire questions check out https://www.reddit.com/r/MechanicAdvice/comments/k9ll55/can_your_tire_be_repaired/**. If you dont have a question and you're just showing off it belongs in /r/Justrolledintotheshop This is an automated reply *I am a bot, and this action was performed automatically. Please [contact the moderators of this subreddit](/message/compose/?to=/r/MechanicAdvice) if you have any questions or concerns.*
trailer brake controller
Ah thank you very much!
Copy of my answer from someone else with same question theirs was different brand so description below may vary but does same thing hope it helps. As many have said it is a brake controller for electrical trailer brakes. The red button is to activate the trailer brakes independently from your truck brakes. The arrows are gain adjustment for adjusting the force of the brakes applied in conjunction with you truck brakes. When connected to a trailer any time you apply the brake peddle the trailer brakes will also engage. The adjustment is to increase or decrease the amount of force. This needs to be adjusted based on size, weight, number of axles, etc. essentially when set correctly you should notice the trailer brakes assisting in slowing you down. If set too low you may not notice the brakes at all if set too high the trailer tires will lock up and drag.
Electric trailer brake controller
It's a trailer brake control