Hello.
1. There are two folder ( flas and flash / skins)in file list. What are they?
2.i know that if i past a file outside of flash folder , it will be deleted after reboot but i dont know how to open flash folder?
3. I select the flash folder and past the file. But after reboot there is no file. I make backup of configurations and after reboot it is not there.
Thanks for your help

If I remember it well you can use the flash/skin folder when you are using Dude.

I don't know which product you have from Mikrotik but most of the modern products don't have much Flash any more and RAM is used to save files. On reboot those files are deleted.
If you want to save your files then you can add an USB stick as and Disk or if your product supports it a SDcard as Disk.