Symbol is the second main reason to use Sketch.
So, what is a symbol? If you’re coming from Photoshop, symbols act like a more advanced smart object. But that “more advanced” make a huge difference.
Let’s take for example a button. You make the rectangular shape and put a background colour (blue) and a border (dark blue). You also add a text layer on top of that rectangular and you type the action (let’s say “click here to login”). Now you select the rectangular and the text layer and convert them to a symbol and the magic starts. These 2 layers will become one. Now everywhere in the interface you use this symbol one of the magic things it will do is something called overwrites. In other areas within the interface, you can use this button but overwrite the text “click here to login” with something else without changing the button. And at any point if you decide that this button should not be blue but green, you go to the symbols page, change the master button and bam, all instances for this button will change to green and they will keep the overwrites you applied.