A seam ripper is a small tool used for cutting stitches.
They often lose their sharp cutting blade and need replaced often.
The Nifty Notions brass seam ripper seems to be the gold star of seam rippers and one to last a lifetime.
I understand it has a lifetime guarantee, and it at any time you need a new blade, you can send it back to the manufacturer and they will replace it just for the cost of the postage.
In use the blade is inserted into the seam underneath the thread to be cut. The thread is allowed to slip down into the fork and the tool is then lifted upwards allowing the blade to rip through the thread. Once the seam has been undone in this way the loose ends can be removed and the seam resewn.