A sapper is a military engineer, The "follow the sapper" bit comes from days of old, when it came around to breaching fortifications the sappers designed the fieldworks to enable the breach. The assaulting troops then had to Follow the Sapper through these fieldworks to get to the breach site. Basically means everyone else has to follow the engineer to get thier job done.

there you go first little bit of british military history for you

Johno