Your direct maternal lineage is the line that follows your mother’s maternal ancestry. With the exception of yourself, if you are male, this line consists entirely of women. It traces your mother, her mother, her mother’s mother, and so forth back to our shared common maternal ancestor. For genealogists, this clear line means that they can trace two or more descendants of a single woman many generations back and compare their mtDNA results with the expectation of a match. For those interested in deeper ancestry, tracing the modern geographic origins of exact matches means that they can discover the origins of their own line.
Note that because mtDNA follows exclusively the direct maternal line, common ancestors between you and your matches on other parts of your tree are coincidental.