Registering with police in the UK can sometimes come at a surprise, However it is not anything to be concerned about.
As per the Home Office's policy guidance, they can request a person to register with the local police force when they are in the UK. If you are chosen to register with the police, you would receive this endorsement on your passport and visa letter when you receive your visa. Keep in mind that this is assessed on a case by case basis.
Upon arrival in the UK depending on the condition imposed you would need to take your passport with the visa stamp and letter to your local police station and register.
The endorsements may vary and could be any of the following:
Pol Reg
Pol Registration
Police Registration within 7 days of UK Entry
Police Registration within 7 days of arrival
Police Registration within 7 days of arrival in the UK
In order to find the police force responsible for your local area, you can find them here by inserting your postcode.