Can someone explain what this piece of code does? string SectionID = ctrl5.ID.Substring(1);

Question

I underastand that the first bit is assigment of value to variable but  "ctrl5.ID.Substring(1);" I am unsure about. 

This has been copied from a web app we have, I need to know how the sectionid variable is getting the value it gets.

string SectionID = ctrl5.ID.Substring(1);

TIA