questions about z(ing) the frame or whatever its called

i'm still new to the whole truck thing i have a neon on bags and this will be my first truck i've done i have an 87 c10 standard cab and bed and i've been reeding on the forums and the say you cut the front of the frame and make a "Z" i guess to make the frame lay even all the way across? Like i said i'm still new if someone could explain this to me as well as post pics that would be great thanks.