This Serato video tutorial explains how to use practice mode, and showcases some vital keyboard controls.
Serato's new 2018 release, Serato DJ, includes Practice Mode that enables DJs to practice their mixing with little else but their laptop and a set of headphones. This wasn't previously available with Serato, and it lowered the entry level for espiring DJs to give the DJ software a try, before having to invest heavily in hardware.