Council Tax Benefit (CTB) is being abolished from next April. In its place will be a range of locally agreed benefits but LAs will only receive 90% of the amount currently paid out as CTB to fund the new systems. As those over working age are exempt from any reduction in the amount previously paid as CTB this means that working age claimants will, on average, see a 20% reduction in their 'new' CTB.
Any sums rebated under the new schemes will not be paid as part of Universal Credit (UC) but LAs are expected to bear in mind the UC paid to applicants.
I'm not sure what the last bit means but I've assumed that it might be that claimants can't receive more than the UC cap.