A chirp is a signal in which the frequency increases ('up-chirp') or decreases ('down-chirp') with time.
The Create Chirp Waveform documents shows how you can use the LabVIEW Chirp VI to create a linear chirp. This examples using a for loop and some mathematical operations to show how you might create a up-chirp.
Download the VI from the attachment (you will need LabVIEW 2009)