mtDNA tells you nothing about a person except perhaps mother lineage over the short term, but that is a stretch. So, mitochondria themselves are thought to be symbiotic prokaryotes living within animal eukaryote cells. There can be up to 1,000-2,000 mitochondria per cell. They have extremely small genomes. There are trends associated with different genomes. It'll seem like a collection of random facts but everything is related.
So, mitochondria have a small genome separate from the genes kept in the nucleus. Small genomes tend to have a high mutation rate. A high mutation rate indicates extreme competition for survival and mitochondria have an extremely small genome. Its actually a separate set of genes within the mitochondria that cannot be imported across the double membrane. Its only a handful of proteins that are coded within the mitochondria. Simpler organisms have fewer genes, small genomes, and high mutation rates. An example of this in the real world is something like crabs. They have 2million viable offspring and maybe 2 or 3 survive.
The opposite of this is vertebrates in general but there is still variation there. You'd think that humans then have some of the largest genomes in the animal kingdom but this isn't true. We have 3b base pairs where as there are animals like salamanders with remote small populations who have 10b base pairs and so are apparently under less selective pressure. Plants can have even more. Their mutation rate is very low and their genomes explode with tons of non protein coding regions.
Any differences in mtDNA are going to be purely superficial. Such as mutations in the 3rd codon which usually doesn't result in an amino acid substitution. It means nothing. Mitochondria don't even follow the codon table, likely to save genome space.
Anyways mind the psuedo-science babbling sources, there are alot of those these days. mtDNA tells you nothing about a person.