It's a formal title of respect used for a married woman, similar to "Mrs." in English. You can also use it as a polite way of addressing any woman, regardless of marital status, similar to "Madam."
Here are some examples:
* Senhora Maria: Mrs. Maria
* Boa tarde, Senhora: Good afternoon, Madam.
* A Senhora está bem?: Are you alright, Madam?
However, it's important to note that the use of "Senhora" can be considered a bit formal in some situations. It's more common to use informal terms like "Dona" for women you know well or "Senhorita" for younger unmarried women.