What is spotlight indexing and should I turn it off?
Leave it on.

You use an OS that heavily depends on this, you need to allow it to index. 

Yes, it keeps track of where all your documents &amp; data are. After any major OS update it will re-index from scratch; the rest of the time it will just periodically tick away in the background, interfering with nothing and helping a lot of the system. 

SSDs are more resilient than people give them credit for. Yes, the more you use it the faster it will wear out, but a bit of file indexing is going to perhaps shorten its life by 20 minutes or so, overall ;-)