Manifest v3 refers to the specification for browser extension manifest files, which enumerate the resources and capabilities available to browser extensions. Google's stated rationale for making the proposed changes, cutting off blocking plugins, is to improve security, privacy and performance, and supposedly to enhance user control.
"Users should have increased control over their extensions," the design document says. "A user should be able to determine what information is available to an extension, and be able to control that privilege."
But one way Google would like to achieve these goals involves replacing the webRequest API with a new one,
declarativeNetRequest.
The webRequest API allows browser extensions, like uBlock Origin, to intercept network requests, so they can be blocked, modified, or redirected. This can cause delays in web page loading because Chrome has to wait for the extension. In the future, webRequest will only be able to read network requests, not modify them.
The declarativeNetRequest allows Chrome (rather than the extension itself) to decide how to handle network requests, thereby removing a possible source of bottlenecks and a potentially useful mechanism for changing browser behavior.
"The declarativeNetRequest API provides better privacy to users because extensions can't actually read the network requests made on the user's behalf," Google's API documentation explains.