Section 8.2.1 The Headline and Footline of the
beameruserguide lists the predefined inserts that can be used to add information to the
footline template. If you want to add some additional information (not covered by the listed inserts), you can use the
text line option for the footline template.
A little example in which "some text" is added as well as the short name of the author centered and the page number flushed to the right (I used a
\parbox with some negative vertical skip to shift the information a little bit upwards):