Something that is chosen or selected automatically. An option that is present unless you change it.
Web browsers (like Internet Explorer) will have a default web page. That's the web page that opens up when you open Internet Explorer. Unless you change it, every time you open Internet Explorer, the default page is going to open.
So to use the phrase by default in that situation... "When I open Internet Explorer, Yahoo.com opens by default."
I would guess that it means that unless you have reconfigured your games console "LK" will be activated by key "A" and "LP" will be activated by "B". In other words those actions are assigned to those keys "by default".