I have a small explanation, and a medium sized explanation. If they ask I'll begin with:
I build websites, my job is making sure websites look as good and are just as easy to use on phone and tablets as they are on a computer.
If they ask for more details about it, I usually explain digital architecture + commerce with an analogy to architecture + commerce they can relate to: a retail store. If asked for more details about my work I'll continue to explain it this way:
Imagine you're a business and a large portion of your customers, maybe 40% or more, are wheelchair-bound. If you make profit by people coming into your store and spending their money, then it makes sense to build a ramp so the extra 40% of your potential customers can get into your store with their money and spend it. I'm like a ramp builder for websites, helping businesses reach more people, and in turn have a real chance of making a lot more profit.
I modify the 'architecture' of online businesses to make them more accessible to customers.