It's pronounced roughly like "so-wa" with a slightly softer "s" sound.
You can use it at the end of a sentence or phrase to make it more polite, similar to how you would use "please" in English.
For example:
* "ཇ་གང་གསོལ་བ་" (ja gang solwa) - "Could you please give me some tea?"
* "ཁྱེད་ཀྱིས་མཛད་གསོལ་བ་" (khyed kyis dzed solwa) - "Please do it."
It's important to note that Dzongkha has a complex politeness system, and the best way to use "solwa" depends on the context and your relationship with the person you are speaking to.