TRIM is a command that allows the operating system to inform the SSD controller which files are no longer in use. In other words, when you delete something from the ssd, the operating system does not erase the actual data from the drive, the OS will just mark that space as free space and overwrite the data when needed.
However, the issue with mainstream SSDs with MLC NAND is that only blocks can be erased, individual pages cannot be. To make this simpler, think page as a file and block as a folder. So, if you want to delete one file (page) inside a folder (block), the whole whole folder needs to be erased and rewritten to get rid of that file. The issue raises when you don't have TRIM because the SSD controller still thinks that the file is in use. When the OS decides to overwrite that file, you first need to read the whole block to the cache (usually DRAM) and then you can rewrite the folder (block). The difference is that if you had an empty block, only the write action would have to be done. Now the SSD needs to read the while block first and then write. For the end-user, that appears as performance degradation. It looks like your write speeds have gone and they can easily degrade by 50%.
With TRIM, the OS can inform when a file has been deleted and the SSD controller can do this action in the background. You won't have useless files floating around, thus write speeds will remain great.
Windows 7 supports TRIM on all SSDs. As of Mac OS X Snow Leopard 10.6.6, TRIM is enabled on 2011 Macs with SSDs. OS X Lion is to add TRIM support for all Apple SSDs. As of DP4, third party SSDs are not supported. This may change in the future though. For third party SSDs, there is
TRIM Enabler, but it is known to cause problems for some SSDs see the
forum thread for more info).
AnandTech's article covers this through with a great example. If you are interested in this whole performance degradation issue, read that, it will open your eyes a lot. In my opinion, the performance degradation issue is overhyped. Most of today's SSDs have very effective garbage collection which helps if your OS does not support TRIM. A good SSD will experience little to no degradation, even without TRIM.