What does "Cambridge CB21 RP" stand for?

It is probably a mail station on the Cambridge campus.

CB21 RP
CB stands for what?
RP for what?

It's probably what in the USA is called a Zip Code and most everywhere else in the world is known as a Postal Code or post office designation. The specific letters and numbers may have some meaning to post office personnel but don't really mean anything to you or me.

It is the post code. You're right.

Everyone is right. The post code defines a small geographic area; in a town or suburb this will typically be as small an area as containing 5 or 6 houses.

Look up any British address: each should include the unique post code.

Error in typing or copying: a space in the wrong place. The correct postcode for the Pitt Building is CB2 1RP. They are placed at the end of a postal address on their own line

Joseph Bloggs
24 Green Street
London
SE24 5QQ

UK postcodes are alphanumeric and between five and eight characters long, plus a single space separating the outward and inward parts of the code. Each postcode unit generally represents a street, part of a street, or a single address.

The 'outward' part identifies first the postcode area, using one or two letters (for example L for Liverpool, RH Redhill and EH Edinburgh). A postal area may cover a wide area, for example RH covers north Sussex, which has little to do with Redhill historically apart from the railway links, and Belfast (BT) covers the whole of Northern Ireland. These letter(s) are followed by one or two digits (and sometimes a final letter) to identify the appropriate postcode district (for example W1A, RH1, RH10 or SE1P). All, or part, of one or more postcode districts are grouped into post towns.

Larger post towns may use more than one district, – for example Crawley uses both RH10 and RH11. In a minority of cases a single number can cover two post towns - for example, the WN8 district includes Wigan and Skelmersdale post towns. Some 'non-geographic' outward codes are used for purposes that are not associated with any particular location.

The 'Inward' is used to assist with the delivery of post within a postal district. The first character is a number denoting a 'sector' and the final two letters identify the postcode unit, which may be a group properties, a single property, a sub-section of the property, an individual organisation or (for instance Driver and Vehicle Licensing Agency) a subsection of the organisation.The level of discrimination is often based on the amount of mail received by the premises or business.