Its slowly starting to make sense

I understand everything so far haha
So the first off spring I get I.E all little brother and sister chicks (will all these come out barred)? If not do I always keep the Cock bird barred when breeding the off spring the couple of times.
You say back cross to the parents? what does this involve? Like 3 years down the line for example I will have all Cuckoo Ko Stags and Hens yeah, then for the 4 year breeding I will breed the 3rd year off springs back to the 2nd year parents that birthed them?
Or am I making this more complicated than it should be haha. I am knew to this so its all very interesting.
Without sounding like a numpty what is the main reason to back breed to parents after so long rather than just continuing to breed offspring down the line?