He kind of defines it in terms of the opposite:

>But early in the day, my mind is hyper-analytical. It’s trying to analyze every word I write from ten different angles. Hence every tweet felt like hard work.

People who just blurt out whatever they think without considering its effects or even how to word it are often said to have "no filter". That is often saying things they shouldn't, things that are inappropriate, or insulting or demeaning toward other people. Or getting the tone all wrong.

Someone who over-analyzes everything before speaking or when writing and can never seem to find just the right perfect words, and therefore just stays silent could be said to be over-filtering.

I read "low-filter" mode as being a middle ground of casual, easy conversation mode. Not no-filter inappropriate, but also not overly analytical and formal and rigorous.

Gotcha, that makes sense, thanks

It's part of the article this comment section is discussing.

The article mentions it a few times, but never explains what it means by the term. It only says that it often occurs when he's a bit tired, but not what it is.