You would need to provide a bigger excerpt than this. I cannot find that text online.

https://www.degruyter.com/california/view/book/9780520907836/10.1525/9780520907836-007.xml?language=en

Gotta dig pretty deep to find the notion of a “mediator” in the opening text of the first Meditation, as posited by Garber.

And with this, the project has begun. Descartes’ meditator quickly
begins by rejecting the commonsense epistemological principles on
which everything he formerly believed rested, and quickly sets about
putting the world back together again. Of course, one of the central
projects undertaken in this connection must be the replacement of the
epistemological principles rejected with new, more trustworthy principles. Just as Descartes’ meditator undermined his former beliefs
by undermining the epistemology on which they were based, he will
rebuild his world by rebuilding its epistemology.

Typo for ' meditation'?

It's a typo for Descartes' "Meditator" - i.e. the guy doing the meditation. It's in the first line of the text in the link supplied by Mww.....