What did he mean by this is a term that falls under the category of shitpost, usually asked either satirically or genuinely depending on the context.
It can be applied as an honest question when somebody encounters a statement online that is hard to comprehend. On the other hand, the question can be also used in context with very simplistic statements to gain ironical or satiric connotations.
What did he mean by this is known as an iconic element of 4chan culture.
The expression started to initially appear on 4chan around 2013, but only gained significant recognition from 2015.
One of the first threads that featured what did he mean by this appeared on the /tv/ (television) board.
The expression online was mostly prominent in various boards of 4chan, for example the /co/ and /mu/ boards. What did he mean by this is a very popular term on these boards for its short nature and easy usage.
In 2015, a clip from the TV show featuring a monologue by actor Aidan Gillen was uploaded on YouTube by the channel BIGBLACKLAWL, titled “what did he mean by this?” The video generated 52.000 views over the years.