B1 English test is an English test that you have to be able to pass if you want to get a spouse visa to settle in the UK. You can book the test in one of the institutions that have been approved by the government of the UK. You can see the list of all the institutions on http://britishlifeskills.com.
It means that if you cannot pass the test, you cannot get the spouse visa. If you do not pass the test, you can still be able to apply for another visa or extend the permission you have in order to stay if you have a child in the UK who is a British citizen or has lived in the UK for seven years that it would be problematic for them to leave the country. Else, you can also apply if there would be difficulties too hard to overcome for you and your partner to live outside the UK.