Sign-in means to sign or write the word "arrived" in the designated book, indicating that I have arrived. That is to say, by recording, I show that I have participated in or participated in an activity. Mostly used for attending meetings or going to work.
There are many ways to sign in. The simple way is to use a pen and paper. For example, teachers in schools ask students to sign their names and count the number of people. There are many ways to sign in electronically, and punching in at work is another form of sign-in to prove a day's work. Some websites and forums have sign-in areas to prove that they have been to the forum and increase the number of clicks on the forum. There are also QR codes and face recognition sign-ins for guest records.