Updated: Jan 16, 2019
PPI is referred as pixels per inch.
In short: It controls the quality of an image, the higher ppi the better.
How use it: When creating a project in Photoshop make sure you set the ppi to 300ppi, since usually it is 72ppi.
What it actually does to your Photoshop project
So lets say your project size is 1000x1000 pixels and your ppi is 72. If you try to export your project in original size then the quality will be fine. But if you try to export it 4 times larger (lets say 4000x4000 pixels) then the quality will be worse.
But now lets say the ppi of your project is 300. Even if the size is 1000x1000 pixels and you export your project 4x times larger, then your project will still have a good quality.
So how does this work?
Since 300ppi is 4 times bigger than 72ppi that means you can export your project 4 times larger and it will still be in good quality.
Just watch the video