The shimenawa marks the sacred place where the gods descended. It is believed that unclean things and evil spirits cannot enter the place where the shimenawa is put, and it also serves as protection against evil. The reason why shimenawa ornaments are hung at the entrance is to show that this is the house that welcomes the god of the year.
The Japanese New Year has many customs and traditions that are truly profound. When I was a child, many more people followed this custom. However, in modern times, it has become simpler and there are many things that people do without knowing the meaning of them. However, I hope that as many of these customs as possible will remain in the future.