The term "ad lib." is usually used in music to mean- "do as you please". You might also see the phrase "repeat ad lib.", which means repeat for as long as you want (or as long as the conductor/ soloist wants). 

This trumpet duet incorporates features of both descriptions of "ad lib.", featuring a repeated 2 note pattern, and also throughout the performance, it returns to the slow, reflective moments that begin this work.

This is a good piece to explore the tonal world of 2 trumpets, while keeping the audience interested and excited.