Complex sentence: TextSecure acts as a normal SMS/MMS app when communicating with non-TextSecure users, but will send messages encrypted over a data channel when the recipient is also a TextSecure user.
Split into simpler sentences: TextSecure acts as a normal SMS/MMS app when communicating with non-TextSecure users. By default, the application will send messages encrypted over a data channel when the recipient is also a TextSecure user.