The Den is a term exclusive to Voltron Force that refers to both the Den Resistance, a pro Voltron resistance against Sky Marshall Wade and the Galaxy Alliance, and the Den, a secret base operated by Pidge, Lance and Hunk in the Galaxy Alliance base on Earth for the sole purposes of gathering information to help Keith in his mission to retrieve the Black Lion.
The Den (Location)[]
A wide shot of the Den
The Den first makes an appearance in the first episode of Voltron Force, New School Defenders, when Lance takes Daniel and Vince through a secret wall panel in the simulator room to it. The three are introduced to Hunk and Pidge, and they quickly reveal their ulterior motives for using it, doing so to help Keith on his secret mission. From there, it becomes a temporary home base for Lance to give orders to the cadets until they retrieve Wade's 4 stripe pin and make it to Arus. Pidge stays at the Den, staying in contact with Keith as he escapes from Wade's base on Tarvos in the Black Lion until he's called to report in to security by Wade.
The Den doesn't make reappear until Clash of the Lions when Daniel and Vince make their way into it in order to better contact Daigo to bring the Lion Riders to help them in the fight against Wade's Mecha Lions. Once completing this, Vince also brings up Pidge's Voltron schematics in order to compare them to the data he got when he temporarily connected to the Wade Voltron. From there, Vince figures out how it works and how to defeat it. This is the last time the Den is seen in the show.
The Den Resistance[]
The Den Resistance members Keith is trying to break out of the Void.
The Den Resistance was first mentioned in New School Defenders, when Keith expresses confusion at Manset's sudden betrayal, stating that he 'was thoroughly vetted by the Den.' Later, in Wanted and Unwanted, it is revealed that Manset actually is a member of the Den Resistance, and helped create the plan for Keith to get in and out of the Void.
The biggest indication that this is an entire Resistance and not just Pidge, Lance and Hunk operating in the Den is in Wanted and Unwanted, when Manset tells Daniel why Keith intentionally got himself captured; to break out some of the members of the Den Resistance.
Apart from these examples, the Den Resistance isn't massively talked about. It is presumed that the members are working in the background to help the Force, at least until Clash of the Lions, where Wade is finally defeated, thus making its existence redundant.