If you're applying for a fiance or spouse visa, and you are is "inadmissible," then you'll probably need to apply for a I-601 Waiver.
There are several reasons why someone might be inadmissible and may not be allowed to come into the U.S., including:
- overstaying their visa/accumulated illegal presence in the US for more than 180 days
- having certain communicable diseases
- having a dangerous physical/mental disorder
- having committed certain crimes
- using fraud or other illegal means to get into the US
- and more
If you need legal advice and want to know whether you'll need a waiver, you should speak with an immigration attorney.