It refers to a review of the 2019 film Joker on the platform of Letterboxd, indicating that their friend Eric also had a phase and became like the Joker. It received widespread forwards and shares on social media making it an iconic review with regards to letterboxd.
The phrase “My buddy Eric” originated from a TikTok trend in which users would record themselves doing odd things with the caption, “Y’all the first to see” or something like that. Eg “Y’all the first to see a buzz lightyear action figure be dipped in chic fil a sauce.” This trend then lead to people commenting things like “My buddy Eric saw this first.”, even though it was very odd. This then became popular in internet culture, and My buddy Eric became a household name.