I know I occasionally reference things in Leftover Soup that might be considered niche knowledge. I strive to make the strip comprehensible, though, to people who may not be familiar with the subjects in question. In this case, I hope it's obvious from context that "wiped" and "TPK" (for "Total Player Kill") refer to Milendra Frostwind, Axle, Birchwood Undertoss and Cishalla Tsettatta being utterly massacred by monsters or villains of some sort.
A "will save" refers to a roll you make to attempt to save your character from suffering the effects of some sort of magical or supernatural effect, such as mind control or illusions. (There are two other kinds of saves - reflex saves to dodge things like dart traps or fireballs, and fortitude saves to shrug off things like disease or poison.) Twenty is a high, but reasonable, target number for a twelfth level party. It should be noted that while Ellen's character may be optimized for a great many things, being a hodgepodge mix of races and classes would, necessarily, make her saves especially lousy.
"Action points" are unique to the Eberron setting, they're little bonus points that players can save up and apply to a roll in times of dire need.
All the D&D stuff in the strip thus far has referred to version 3.5, which was a very popular version of the game. Fourth edition came out in 2008, but many players continue to use 3.5 (some due to familiarity and nostalgia, others because they'd already made a substantial investment in 3.5-compatible products). I suppose, as the gulf between strip time and real time gradually increases, Ellen's group will retroactively become more and more old school.
D&D was originally produced by a company called TSR in 1974, then it was purchased by Wizards of the Coast in 1997, which was then later purchased by Hasbro in 1999.
"Strawberry Shortcake", in this case, does not refer to the dessert (which would not be capitalized), but to a cartoon and toy series created by a company called American Greetings back around 1977. Hasbro acquired the rights to it back in 2009, although it is worth noting that Ellen isn't necessarily aware of this relatively recent transfer of intellectual property, and, in fact, assumed Hasbro owned SS from the get-go.
The "Easy Bake Oven" was a working toy oven originally developed by Kenner, yet another company which was purchased by Hasbro in 1991. It was powered by a 100 watt lightbulb (later versions had a dedicated heating element), and allowed children to make real edible cakes that were about the size of a hockey puck.
I think that about covers everything.