You only get so much money a year to put in your ISA. That counts as your ISA allowance for that year.
If you take money out of a previous years ISA you "lose" the ISA allowance for that year.
You can put money back in to your ISA, but it will count towards THIS years ISA not the previous years.
Same as if you want to transfer money from an "old" ISA in say NatWest to an ISA in say Santander. If you take the money out the old NatWest ISA to put it in Santander you lose the ISA connection for that money.
The only way to transfer it is to to go Santander and ask them to do an ISA transfer and NatWest send the money to Santander (you never get to see it) so you "keep" the ISA connection for that money.