What exactly is a mixed anxiety and depression disorder?

''Mixed depressive and anxiety disorder

DefinitionMixed depressive and anxiety disorder is characterized by symptoms of both anxiety and depression more days than not for a period of 2 weeks or more. Neither set of symptoms, considered separately, is sufficiently severe, numerous, or persistent to justify a diagnosis of a depressive episode, dysthymia, or an anxiety and fear-related disorder. Depressed mood or diminished interest in activities must be present accompanied by additional depressive symptoms as well as multiple symptoms of anxiety. The symptoms result in significant distress or significant impairment in personal, family, social, educational, occupational, or other important areas of functioning. There have never been any prior manic, hypomanic, or mixed episodes, which would indicate the presence of a bipolar disorder.'' https://www.ncbi.nlm.nih.gov/pmc/articles/PMC50971...

So basically anxiety and depression which are too mild, too short lived or too infrequent to be considered actual anxiety or depressive disorders.

I'd say this is a good question for your next therapy session

i've never heard of that diagnosis either but i'd say that most cases of Depression are rooted in the consequences of harboring high Anxieties so it makes complete sense to see them together. 

A lot of times you see weird diagnoses is so the insurance covers "I don't really know, it's something not otherwise specified in the DSM". 

i'd guess in your case it means "doesn't meet specific diagnostic criteria for either diagnosis but we need a qualifier for insurance coverage".