search - What is the meaning of "must match" in Solr? -

i have requirement modify must match parameter in solr.

 <str name="mm"> 2&lt;-1 5&lt;-2 6&lt;50%  </str> 

i'm not able understand logic behind syntax.

mm stands minimum should match parameter.
helps configure number of query terms should match document returned in search results.
can configure fixed number or percentage of number of query terms should match.

for detailed explanation , example check link