Traveler's checks are a secure way to carry money because they can't be cashed without two matching signatures. You sign them for the first time in the presence of the person who sells them to you. This original signature is the proof that the checks are yours.
When you are ready to cash a traveler's check, you countersign it by signing it a second time. Because it is identical to your original signature, the second signature proves that you are the same person who purchased the checks. If the second signature on the check does not match the first one, a bank will know immediately that the check has been stolen.
Travelers sometimes sign the second signature in advance, to save time. That is a mistake, because, once countersigned, a check is no longer protected. It is the same as cash. To maintain protection, always wait until the actual moment you cash the checks before countersigning them. Signing in the presence of the person who is cashing the check is proof positive that you are the same person who originally purchased it.