What you can do
The one ISA per type per year rule allows you to only subscribe to one ISA of each type per tax year. For example, in tax year 22/23 you can split your annual ISA allowance of £ 20,000.00 across a cash ISA, a stocks and shares ISA, a lifetime ISA(LISA) and an innovative ISA. Hence you can, in theory, use your £ 20,000.00 allowance on 4 types of different ISA products in the same tax year.
What you cannot do
You cannot subscribe to 2 ISAs of the same type in any one tax year. For example, you cannot split your annual allowance between a cash ISA with provider A and another cash ISA with provider B in the same tax year.
Subscribing across different tax years
If you have subscribed to an ISA in a previous tax year, then there is nothing to stop you opening a new ISA of the same type with another provider in the following tax year, but you can only subscribe to one of those ISAs in the current tax year. For example, if you subscribed to a stocks and shares ISA in tax year 20/21 with provider A, then you could open a new stocks and shares ISA in 22/23 with provider B, without closing or transferring the old stocks and shares ISA. But, once you subscribe to the new ISA, then you are not permitted to subscribe to the old stocks and shares ISA in that same tax year. This would be an invalid subscription.