Letters testamentary are the court document that proves you have legal authority over a dead person's estate. The probate court issues them to the executor after accepting the will, and until you can show them, a bank won't give you access to accounts that were in the person's name alone.[3] If there's no will, the court issues letters of administration instead, and the document does the same job.
The will alone isn't enough. Being named executor gives you no power until a court appoints you, and the letters are the proof of that appointment.[3] People learn this at a bank window, holding a will the bank won't act on.
This page covers the document itself: what it is, exactly how you get it, what courts charge, how long you'll wait, and what to do in the meantime. For the whole court process around it, read the MORTL probate guide.
