Undo move files from the "all my files" to the trash folder
Do I risk to delete these files?


You're fine as long as you don't empty the Trash.


  How can I undo this?


Command ⌘-Z will undo the last action in Finder. So if you just deleted them you can use this shortcut to undelete them all.

Otherwise:


Click on the Trash icon in your Dock to open the Trash view in Finder
For each file in the Trash, right click on the file and select Put Back from the menu (unfortunately you can't issue the "put back" instruction to multiple files at once, see this answer for a way to do it in groups of files that might work or this one if you're comfortable running some AppleScript)