What is 'persist' in TWRP while taking a backup?

hey guys, I just bought OPX 2 days ago and rooted and unlocked the bootloader yesterday. I also removed few apps like slides, doc etc and everything was working fine. Today when I saw storage was almost full (because of those google apps updates) I tried to integrate them into ROM using Titanium backup. 2 of 18 apps were successfully integrated (don't know which) but then it stuck for a lot of time. So i forced stop tit and tried integrating each app individually, but still same problem and it had f**ked system a little so fortunately I had restored the backup which I took before integration. During backup, I selected all the partitions (or whatever it is) and started restoring. Everything went well until the 'persist' was getting restored. Wiping of Persist was successful but it was stuck to restoring for a really long time so aborted the restoring, wiped everything and restored everything except persist. So my question is What is Persist and will it affect my phone 'cause I didn't restored it? and why are apps not getting integrated into ROM? any solutions will be helpful
(and extremely sorry for super-long post :P )

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