PV1 Citrano,
Sure so "fill in" basically means to fill a billet temporarily.
For example, let's say that Havoc 1-1 is doing squad training, and their AR from Alpha is away (sick, IRL things, etc).
In Teamspeak's Global Chat, you'll see a call go out "Havoc 1-1 requesting 1x 11B fillin".
When you see that, go ahead and join Havoc 1-1's channel, and an NCO will assign you to a place "PV1 Citrano you'll be AR for Alpha".
Once you complete OSUT, you'll be assigned to your primary billet (this is explained more if you pass your Day 4 test) - this primary billet is the position you "must" fill when events come along, so your weekly squad trainings etc, and anything other than these "required" events are "fillins"
I hope that clears things up but if you still have questions - let me know!
- SSgt Hardman