The /u/0 just means you're using multilogin. For example, if you log in as foo@gmail.com, and then log in as bar@gmail.com via multilogin, gmail.com/u/0/inbox/w

The /u/0 just means you're using multilogin. For example, if you log in as foo@gmail.com, and then log in as bar@gmail.com via multilogin, gmail.com/u/0/inbox/whatever will take you to foo's email, and gmail.com/u/1/inbox/whatever will take you to bar's. You can thus have adjacent tabs accessing both accounts simultaneously. Also, /u/0 is the default; you can remove it from the URL and it'll continue working just fine.