Brick and mortar in its most simplest usage is used to describe the physical presence of a building(s) or other structure. It's a concept usually referred to in business, which applies to the physical location for a business or organisation.

Brick and mortar business

The term brick and mortar business (bricks and mortar business or B&M business) is often used to refer to a company that possesses a building or store for operations. The name is a metonym derived from the traditional building materials associated with physical buildings — bricks and mortar

Mortar (masonry)

Mortar is a workable paste used to bind construction blocks together and fill the gaps between them. The blocks may be stone, brick, cinder blocks, etc. Mortar becomes hard when it sets, resulting in a rigid aggregate structure. Modern mortars are typically made from a mixture of sand, a binder...

. Its first use was in 1992.

More specifically, in the jargon of online eCommerce businesses, brick and mortar businesses are companies that have a physical presence and offer face-to-face customer experiences. This term is usually used to contrast with a transitory business or an internet-only presence, such as an online shop

Online shop

Online shopping is the process whereby consumers directly buy goods or services from a seller in real-time, without an intermediary service, over the Internet. It is a form of electronic commerce...

, which have no physical presence for shoppers to visit and buy from directly, though such online businesses normally have non-public physical facilities from which they either run business operations from, and/or warehousing for mass physical product storage and distribution.

An example would be the movie-rental shop Blockbuster Video, which has physical stores and is in competition with the newer online rental services offered by Netflix

Netflix

Netflix, Inc., is an American provider of on-demand internet streaming media in the United States, Canada, and Latin America and flat rate DVD-by-mail in the United States. The company was established in 1997 and is headquartered in Los Gatos, California...

. In this sense, the term is also a retronym

Retronym

A retronym is a type of neologism that provides a new name for an object or concept to differentiate the original form or version of it from a more recent form or version. The original name is most often augmented with an adjective to account for later developments of the object or concept itself...

in that most stores had a physical presence before the advent of the Internet. However the term is also applicable when contrasting businesses with physical presence and those that operated strictly in an order-by-mail capacity, pre-Internet.