It’s a style of advertising where you make direct or implied comparisons with a competitive product or products. It’s popular for example with budget supermarkets, comparing for example their price advantage on a tin of beans or similar. In this type of example, it can work well, it is a simple message and the products are interchangeable. Importantly, it is easily understood so is unlikely to upset the Advertising Standards Authority (ASA).
Crucially, to work, what is claimed, must be irrefutably true. For sure, it will rattle the cages of the targeted competitor, who you can expect to retaliate.
For many years, some countries restricted or even banned such advertising. Now, within the EU, it’s allowable, but under close scrutiny from the standards enforcers.