A soapland is a brothel. You pay a woman to use her naked body to clean you. She gets you all foamed up, uses her boobs and ass like a sponge and then fulfills the rest of your agreement.
If all this is making you reconsider your opinion of prostitution then you certainly aren't alone.
Starting out as a remedy to the illegality of blatant prostitution, soaplands called themselves "Turkish Baths" and pretended nothing funny went on. Some Turkish guy got mad about the stain on the impeccable reputation of his nations bathing and forced a name-change. Did he try it first, I wonder?
racial-based rejection. Something about gaijin is a no-go for a lot of these girls, so be ready for disappointment. I can't give you any first-hand experience, but there are plenty of websites out there to help you find a fitting young lady, and also tell you the hotspots to go looking should you so desire.
I haven't heard of any switched-sex arrangements (i.e. man bathing woman), but an open-minded lady can reportedly get herself a same-sex scrubbing.
A very good and often-cited book about Japan's scorching-hot erotic underworld is "Japan's Sex Trade: A Journey Through Japan's Erotic Sub-Cultures" by Peter Constantine. Well worth reading if you have a genuine interest in the secret life of Japan.