This would be referring to the Subtract tool on Box Select:
In later versions of Blender, these text boxes were replaced with icons. Subtract is the 3rd one along:
Personally I think you are wasting your time using any old version of Blender.
I think Sprinse was referring to the older version of the Blender course. Personally I recommend it – the style of the newer Blender courses has never worked for me, and I never would have got into Blender if I took those courses as an introduction today. I still sometimes go back to Michael and Rick’s videos to refresh my memory on things. But I agree, if someone is doing the old course they should be using the later versions of Blender. Sometimes it just means finding where a button has moved to.
(And welcome to the community Sprinse, nice to have you here. )