"We guarantee that you will live your entire lifetime, or double your money back."
-- From Acme Klein Bottle's Unconditional Guarantee.
A lifetime guarantee is a promise from a company or vendor that their product will last... a good long while. What exactly a lifetime guarantee covers is highly variable. Usually there will be some fine print explaining exactly what is covered and for how long; sometimes there is not. When in doubt, ask. But as a general guideline:
For smaller and stand-alone products, a lifetime guarantee usually covers the lifetime of the product. The company has determined how long the product should last, and if it doesn't last that long, you are entitled to either a replacement or your money back.
For products that are either very durable (such as jewelry) or attached to another product (such as car parts), a lifetime guarantee may be given for 'as long as you own the product'.
For either of these two types of guarantee, selling the product generally voids all guarantees and warranties. If you want to have any hope of making a claim on a guarantee, you better have kept your receipt.