SSDs use flash, which by nature have a limited number of erase-write cycles. Once those are exhausted, an SSD basically becomes a read-only device.
The SSD uses techniques to mitigate that, something called wear-leveling; distributing the writes across the drive evenly so that no one portion of the drive wears out too fast.
SSDs also have other things that lead to something called
write-amplification. Because each portion of the drive that is over-written to must be first erased before being written to, and because the size of block that can be erased is bigger than the size of block that can be written, this leads to the drive having to move files around all the time to recover space from deleted files.
The more active the drive is in doing all these things, the closer the drive will be to its theoretical maximum performance, and the more it'll sustain that over its life. The tradeoff is write-amplification, where a single write (save your file) will cascade into far more writes as the data is moved around, metadata is moved, etc etc.
For the most part it's not really a big issue currently, modern drives could take gigabytes being written to them daily for 5 years before it becomes an issue.
But it's going to become more an issue, since the smaller the transistors on the flash, the fewer erase-write cycles it will take before becoming read-only.. and the only good way for them to reduce the cost of these drives significantly is to put more transistors in a smaller area (so you get more memory per physical size of chip, or more chips on each chunk of silicon bringing costs down). So this'll become more of an issue as we go on.
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