Those are "session" files used to provide "state" information for web applications.
This is not an indication that someone is using your your account; the /tmp "directory" is actually a link to /usr/local/tmp, which is shared by many users for storing temporary files and is owned by root.
Because root is the owner of the directory, and because of the way permissions are set, you cannot delete any of these files that you are not the owner of. You can see who the owner of each file is by looking at the directory with the ls -la command, and note the user name and group that the file belongs to.
This is "normal" for a unix environment, and is not a security risk as only "root" and the owner of these files can read them or write to them (you can see the permissions of each file with the ls -la command as indicated above).