Tweens are a way a developer can slowly advance from one property position to another smoothly then instantly.
For Example, If I had a Part that was Transparent (Transparency property is 1), and I wanted to change it so it wasn’t Transparent (Transparency property Wanted 0), I could just do something Like Part.Transparency = 0
However, it does this instantly, which I don’t like. I want it to fade in, because it looks whacky just showing up from nowhere… What do I do? I use Tweens! Tweens, allow for this smooth transition over a time period.
So instead of the Transparency property going from 1 to 0 instantly
Tweens make the transparency property go from 1 to 0.99 to 0.98 to 0.97 etc. until it reaches 0, the end goal, And this Time is controlled by me, tweens take the time to reach the end goal.
Now Tweens work on properties more then just numbers (Numbers are just easiest to understand in my opinion)
Here are some Roblox Docs info on Tweens & TweenService:
Also here are some YouTube videos that might help understand the coding part of tweens.