Xtail watches the growth of files. It's like running a "tail -f" on a bunch of files at once. It notices if a file is truncated and starts from the beginning. You can specify both filenames and directories on the command line. If you specify a directory, it watches all the files in that directory. It will notice when new files are created (and start watching them) or when old files are deleted (and stop watching them).

Note that xtail isn't a graphical (X11) program, it runs on a plain tty. The name likely comes from "eXtended tail" or such.