Pension Credit is a means tested benefit. It tops up existing income to give State Pension age claimants i.e. those age 66 or over (and 'protected' mixed age couples) a minimum level of income for their living costs:

Extra help is available to claimants who are a carer, severely disabled, responsible for a child or young person and/or have certain types of housing costs to pay - such as ground rent or leaseholder service charges.

Pension Credit is separate from the State Pension. It is not based on National Insurance contributions and has no upper savings limit.