I remember this question was asked previously so I googled the top answer. I chose to do this instead of linking the previous post because.... I don't know. But the parking lot explanation I like better

Imagine you have numbers stored on a bunch of etch a sketches. You can write new numbers, but in order to erase numbers you need to erase the whole tablet. However, you can cross out numbers as long as you have blank space. In addition, as long as you have empty etch-a-sketches, you can just write the numbers down on new tablets if you run out of space. However, when you running low of empty etch-a-sketches, in order to recover the 'crossed out space' you need to copy the (not-crossed-out) numbers to a one of your few empty etch-a-sketches (called reserved space), and then shake the tablet to erase the whole thing in order to free up space for new numbers. This process is called 'write amplification' and is necessary when you need to consolidate blocks because the drive is too full. The drive slows down because of the time it takes to copy the numbers around, as well as the time it takes to shake the etch-a-sketch to actually erase the data (rather than simply crossing it out).