A wall sit is where you sit against a wall with your knees at right angles, back flat against the wall and see how long you can last. I've also seen it done where you balance on one leg and hold the other leg out straight. But that's probably not going to be me any time soon. It measures leg strength. From my test, it would appear I don't have any!
This is how it should be done (photo from Ola Fitness blog here).
Apart from my knee being a bit sore (I'm taking my rest day today instead of Sunday although it feels a bit early!) I have been enjoying cooking some different recipes and really thinking about what I eat. First day's workout left me a bit stiff in a few places, but not too bad. So I can cautiously say that I am doing this - taking baby steps - but just following the program and seeing what will become of me, as per my commitment to do just that.