It's a change to the browser that gives site-owners the ability to require a positive attestation of non-modification before running. The stated goal of this change is to make it more difficult for end-users to block ads. As the spec states, blocking ads violates the deal you make with content creators to use your attention to ads as a form of payment.
In practice, this will make it harder, but not impossible, to run ad blockers. Now instead of just finding and installing a plugin, you'll have to first find and install a forked browser that implements the attestation as something like 'return true'. This will predictably decrease the number of people blocking ads.
Personally, I don't object to this. The easy solution for most people is simply: don't consume the content. Or pay money instead of watching ads. Content creators, it must be said, also have the option of self-hosting and/or creating content as a hobby rather than a career. As someone who has grown more and more despairing of any paid-for speech, especially by ads, I welcome this change.
Far more troubling is the possibility of attestation for "important apps" like banking or government. In general this mechanism gives the org a way to prevent you from doing what you want with your data. For example, they can prevent you from scraping data and automating end-user tasks. This takes away your degrees-of-freedom, and using a modified browser will certainly become an actionable offense. In my view this is by far the more troubling aspect of this change, since it take away significant aspects of user autonomy in a context where it matters most.
Technically sophisticated users will note that it's not possible to secure a client, and foolish to try. This misses the point. These changes stochastically change behaviors "in the large", like a shopping center that offers two lanes in and one lane out, or two escalators in and one out. This represents a net transfer of power from the less powerful to the more powerful, and therefore deserves to be opposed.
EDIT: please don't downvote, but rather reply with your objection.