Lore (sometimes stylized as LORE or L O R E) is a meme within the SiIvaGunner community. It referenced in the question "Is this lore?", which is asked sarcastically about anything that seems abnormal, not just actual "lore".
The word "lore" refers to the information about traditions, knowledge, and customs about a certain group. Many different fanbases, such as Star Wars and Lord of the Rings, study and theorize about the "lore" to uncover the often obscure (and ridiculous) background of their obsessions. "Lore" is also used as a synonym of "story".
On SiIvaGunner Edit
As a bait-and-switch channel, SiIvaGunner did not have a plot beyond some minor kayfabe with the channel posing as GilvaSunner, and playing a character of the same name.
Then, SilvaGunner: Rebooted happened. With this story arc, fans of the channel were swamped with questions, some of which appeared to have answers lying in the channel's previous videos: "Futari Happiness (NOZOMI Mix) - Love Live! School idol festival", and the "Megalovania" rips with Chad Warden. While these rips were originally one-time jokes, they were repurposed to make a coherent storyline for SilvaGunner: Rebooted, thus setting the precedent that any rip could be "lore".
This continued even after the end of SilvaGunner: Rebooted, mostly concerning The Nutshack and Mr Rental; however, neither of these were truly lore, and the ridiculousness of it all eventually became apparent over time with the team poking fun at the fans in rips like "Ancient Lore - Demigod" and "Circus (Lore Mix) - Five Nights at Freddy's", as well as placing Misha flipping the bird on the channel banner, only to remove the lighthouse entirely an hour later. In addition, the controversial Green de la Bean rips and subsequent speculation about the Channel Ending took fan's attention off of supposed "lore".
Starting in Season 2, The SilvaGunner Christmas Comeback Crisis would become the main source of all lore on the channel. This time, the lore was more upfront: It didn't raise questions like before, and wasn't hidden in normal rips. This made the question "Is this lore?" mostly obsolete, however it is still sometimes asked sarcastically on shitpost rips that stray from the usual style.