Its like a state of mind.
"I see youve found my underground,
Help yourslefs to guns and ammo"
This is like saying I know youv'e got inside me, but just take whatever you want or need.
"Nothing here has ever seen the light of day
I leave it in my head"
This is like just verifying that it is in her head, but this person has found a way in.
The whole "First day of the rest of your life" Is just like saying now youv'e been in my world (head) your stuck with me, and its great! Its your new life, being with me.
Its like a state of mind. "I see youve found my underground, Help yourslefs to guns and ammo" This is like saying I know youv'e got inside me, but just take whatever you want or need. "Nothing here has ever seen the light of day I leave it in my head" This is like just verifying that it is in her head, but this person has found a way in. The whole "First day of the rest of your life" Is just like saying now youv'e been in my world (head) your stuck with me, and its great! Its your new life, being with me.