I was not gone from there but 1 day Are you saying that my stuff will be back there after 10 days?
It means that after 10 days, every container will reset, removing everything it contains.
If you want to keep your things safe, for saying an example, you can come back to the fort every like 9 days, or less, and your stuff will not reset. If you, instead, are the type who travel much and rarely visit the fort, you better store your things in one of your homes. Also, I'm not sure about that 1 safe place in the fort, so you better not trust because who knows if it's a bug or what. (not saying that that tip isn't right, though!)
And remember, containers are tied to cells (room), if you stored your things in a container inside Fort Dawnguard, everytime you will enter from the main gate, the respawn time will reset. But there are are also the towers outside the fort, so if you stored your containers up there, that cell is tied to Dayspring Canyon, not Fort Dawnguard.
Hope this helped and wasn't much confusing
Love your avatar.