There's no such thing as absolute speed in the universe. But there is relative speed. That's how fast something is going, from something else's point of view. The speedometer on your car measures your speed relative to the road. But another car on the road next to you would say your speed is 0, because from their point of view you aren't moving. That is to say, you're going the same speed.

We used to think relative speeds just added or subtracted together normally. The same rules you learned in math class. But Einstein figured out that isn't true. See, Einstein and many others knew that the relative speed of light is always the same. No matter how fast you're going, light is faster. And always by the same amount. You can never get closer to the speed of light. It didn't make sense to anyone until Einstein figured it out.

Einstein realised that the faster you're going, the slower time passes. So even if you're going at a million miles an hour, you just slow down, and now from your fast/slow point of view, light is still beating your speed by the same amount. You don't experience time as slower, but anyone looking at you would see you moving in slow motion.

That's how drag's high school physics teacher explained it to drag. Drag oversimplified a bit, but all the important bits are there, and anyone could figure out the rest if they spent the time thinking about it. Anyone who thinks relativity is hard to explain doesn't understand it. That's what Einstein was saying.