ok, i will try to explain this the best i can.
Okay, so we know that in the first shell there are 2 electrons. Only 2 electrons can fit into the first shell.
Then we have the second shell which can fit only 8 electrons.
Then, lets say we have a third shell. The third has one electron in the valence shell. The ion is "unhappy/unbalanced". We need to remove that electron to make the ion balance. So we need to remove that one electron and transfer it to another ion to make that ion "happy/balanced".
hope this helps you understand.