max_df is used for removing data values that appear too frequently, also known as "corpus-specific stop words".
For example:
The default max_df is 1.0, which means "ignore terms that appear in more than 100% of the documents". Thus the default setting does not ignore any terms.
min_df is used for removing terms that appear too infrequently.
For example:
The default min_df is 1, which means "ignore terms that appear in less than 1 document". Thus, the default setting does not ignore any terms.
Hope this answer helps.