It codes for structures essential for the function of the mitochondria itself.
Generally speaking, the mitochondrial DNA contains 37 genes that code for the following:
1) NADH Dehydrogenase
2) Cytochrome b subunit of the Coenzyme Q
3) Cytochrome c oxidase
4) ATP Synthase
For further in-depth reading & references:
Genetic Home Reference
Jan-Willem Taanman, The mitochondrial genome: structure, transcription, translation and replication, Biochimica et Biophysica Acta (BBA) - Bioenergetics, Volume 1410, Issue 2, 9 February 1999, Pages 103-123, ISSN 0005-2728
I hope all your questions were answered.