From MothCraft

It adds a new item to your inventory which you can use to save your inventory. For ex: you die, if you have the SaveRod in your pocket it will prevent the items to drop. You can have as many saverods as you want, but every time you die, it cost you 1 SaveRod.

When you die

When you die and you got the SaveRod your items will be spared. So you don't lose your inventory. When you are out of rods and you die, the default stuff happens, you lose your stuff.