In GTA Online, you usually move around by car. In order to always have a quick access to a car without resorting to theft, which may often result in a police pursuit, it is good to mount a tracker in your car. Once you buy it for your car, it will always be waiting nearby when you log into the game. This is a major convenience, especially when you spawn in the middle of wilderness, where there are hardly any cars to use. To have a personal car, first you need to find a car you like. In other words, you have to steal it. Once you get a car, head to one of the Los Santos Customs shops - they are marked on the map with a spray can icon.
When you enter a LSC garage, you will see a list of car modifications. To mount a tracker, choose option Loss/theft prevention. This will roll out another list with two options. Click on Tracker option. This is it. Now you can always find your car. It will be marked on the map with a car icon. The first tracker is mounted for free, every other costs 2000$.