An Omamori is a small amulet that can be bought after a prayer in a Buddhist temple or Shinto shrine. It is a small cloth bag that contains a prayer or sacred inscription. Its name comes from the verb "mamoru" which means to protect. It is therefore protection for specific situations: health, marriage or good fortune.
It is therefore a good luck charm, so indispensable in the lives of Japanese people that they often have one or two. They keep them in their wallet, hang them up in the house, the car or even on their phone.
The omamori are often brightly colored, and bear the name of the temple from which it comes on one side, and the area it protects on the other. Inside is a small piece of paper on which is written a prayer and the name of the god who will protect you. But beware, if the cover is open, it will no longer protect you and you will have to buy another one.
These amulets don't really have an expiration date, as long as they are in good condition and their inside is never exposed to the air, they will keep their qualities. But it is customary in Japan to replace them every year. The old ones are then returned to the shrine and destroyed. But it is not an obligation and many people keep them because they evoke a special memory.
One of the reasons for the success of omamori is that they are inexpensive (a few hundred yen). On the other hand, they have been able to evolve and adapt to today's world. This is the case for "safety on the road". Similarly it has become popular in some stores to offer generic omamori with the effigy of certain characters such as Mickey Mouse or Hello Kitty.
The interest is still to buy them in a temple or sanctuary. You make an offering there, you invoke God and you with also a great souvenir for yourself or for friends. It is also a significant source of income for these places which are not subsidized by the State. So if you liked the atmosphere of a temple or a sanctuary, buy an omamori and you will make one for the community and something good for you ;-)