Often new PCs come with pre-installed software which the manufacturer was paid to include but is of dubious value to the purchaser. Most of these programs are included without the user's knowledge, and have no instructions on how to opt-out or remove them.
Pre-installed software is software already installed and licensed on a computer or smartphone bought from an original equipment manufacturer (OEM). The operating system is usually pre-installed, but as that is the rule, generally this term is used for additional software from the bare minimun, from other sources (or the operating system vendor).
Unwanted pre-installed software (also known as crapware) can include major security vulnerabilities, like Superfish, which installs a root certificate to inject advertising into encrypted Google search pages, but leaves computers vulnerable to serious cyberattacks that breach the security used in banking and finance websites.
Some free download websites use unwanted software bundling that similarly installs unwanted software.