What does "~mitochondrial DNA ~bp linear DNA" means?
Well, Mitochondrial DNA CAN be linear (in some organism), see the wikipedia page on this 


  In most multicellular organisms, the mtDNA - or mitogenome - is
  organized as a circular, covalently closed, double-stranded DNA. But
  in many unicellular (e.g. the ciliate Tetrahymena or the green alga
  Chlamydomonas reinhardtii) and in rare cases also in multicellular
  organisms (e.g. in some species of Cnidaria) the mtDNA is found as
  linearly organized DNA. Most of these linear mtDNAs possess telomerase
  independent telomeres (i.e. the ends of the linear DNA) with different
  modes of replication, which have made them interesting objects of
  research, as many of these unicellular organisms with linear mtDNA are
  known pathogens.[19]
  
  For human mitochondrial DNA (and probably for that of metazoans in
  general), 100-10,000 separate copies of mtDNA are usually present per
  cell (egg and sperm cells are exceptions). In mammals, each
  double-stranded circular mtDNA molecule consists of 15,000-17,000[20]
  base pairs. The two strands of mtDNA are differentiated by their
  nucleotide content, with a guanine-rich strand referred to as the
  heavy strand (or H-strand) and a cytosine-rich strand referred to as
  the light strand (or L-strand). The heavy strand encodes 28 genes, and
  the light strand encodes 9 genes for a total of 37 genes. Of the 37
  genes, 13 are for proteins (polypeptides), 22 are for transfer RNA
  (tRNA) and two are for the small and large subunits of ribosomal RNA
  (rRNA). This pattern is also seen among most metazoans, although in
  some cases one or more of the 37 genes is absent and the mtDNA size
  range is greater. Even greater variation in mtDNA gene content and
  size exists among fungi and plants, although there appears to be a
  core subset of genes that are present in all eukaryotes (except for
  the few that have no mitochondria at all). Some plant species have
  enormous mtDNAs (as many as 2,500,000 base pairs per mtDNA molecule)
  but, surprisingly, even those huge mtDNAs contain the same number and
  kinds of genes as related plants with much smaller mtDNAs.[21]


https://en.wikipedia.org/wiki/Mitochondrial_DNA

In the case of your sequence, it only means that it has been entered in NCBI as a linear piece of DNA (it is usually the case when you sequence only a piece of a circular DNA for example)
It does not mean that within the organism it is linear ! 

I hope this answers your question !