This is the 13335th most frequent Russian word.
мыльная
"Мыльная" means "soapy" in English.
Here, 'мыльная' ('soapy') is an adjective describing the water, indicating it contains or is related to soap.
Я использую мыльную воду для уборки.
I use soapy water for cleaning.
In this sentence, 'мыльная' ('soapy') is an adjective connected to 'surface', suggesting the surface is slick or covered with soap.
Рабочие аккуратно обработали мыльную поверхность.
Workers carefully treated the soapy surface.
Here, 'мыльная' ('soap') is part of the compound noun 'мыльная опера' ('soap opera'), used figuratively to refer to serialized shows, originating from the soap advertisements during such shows.