The phrase "extra foam" refers to adding more bubbles or foam to something, usually in a drink like coffee or a milkshake. It means you want more airiness or frothiness in your drink. Full definition
I can't wake up without having my morning coffee with extra foam - I crave it! (fitreserve.com)
With pocket spring support, extra foam for squishiness, and playfully sophisticated grey or turquoise chevron upholstery, the Pop Mini Chair is a perfect place for your mini-me to relax. (modernnursery.com)
Therefore additional surfactants that are able to produce extra foam are often used as foaming agents. (scientificamerican.com)