> What is an "algorithm"?

To you and me it's a very wide category, and there may be hundreds or thousands or even millions involved in me simply typing this, it being saved somewhere and the served back to you.

For lay-people though, it's become a term which means "The thing which measures your preferences and usage patterns then decides what to show you". So for lay-people to want to turn off the algorithm, they tend to mean going back to "just what your friends posted, in order"

> When does an algorithm become harmful?

I think this is a research subject. AFAICT it's pretty certain now that social media 'algorithms' have been optimised for addictiveness, which is likely to overlap somewhat with what users find interesting, but also cover things they find mildly titillating and/or outrageous.

The harms come in a bunch of forms. The never-ending scrolling of bite-sized videos seems massively destructive to attention span, and people seem to get addicted to it and may neglect other parts of their life. The platforms also seem really keen to send men and boys off to pretty hateful 'manosphere' content which is apparently making life hell for women (including teachers of teenage boys). Then we have all the body-image and anxiety issues caused in teenagers constantly exposed to influencers showing off their (faked) wealth, beauty etc.

Switching off the "shoving this all in your face" part of social media seems to me to be a win. When it was actually a way to keep in touch with friends and family it was a different game.