The process whereby an advertiser purchases ad space directly from a Publisher, without the use of AdTech platforms (Ad servers, Demand-side platform (DSP)s, and Supply-side platform (SSP)s).
This was how ads were purchased in the early days of online advertising.
The opposite of manual media-buying is Programmatic media-buying, which includes many different types of methods, such as Real-time bidding (RTB), programmatic direct and private marketplaces (PMP).
Even though many large Publishers and advertisers still buy and sell ads via their sales teams, many of them use programmatic platforms such as Ad servers to automate the delivery and reporting process.
Related post: Understanding RTB, Programmatic Direct and Private Marketplace