For a dependant relative (eg father, mother) to enter the UK and stay for more than six months they will require a Family of a Settled Person Visa. A Standard Visitor Visa is fine if you want to stay for less than six months.
To apply for a Family of a Settled Person your dependant relative must have a family member living permanently in the UK who is either a British Citizen, has settled in the UK, or who has asylum or humanitarian protection in the UK.
It usually takes about 12 weeks to get a decision and they may have to pay the health surcharge (which entitles them to medical treatment from the National Health Service).
With a Family of a Settled Person Visa they can work in the UK. They may be able to extend their visa and settle in the UK permanently (Indefinite Leave to Remain).