know a thing or two is an idiomatic expression generally used when someone is not giving credit for a person's knowledge or skill.
Jane, please. He does know a thing or two about plumbing. Let him do his job.
It means that the person knows quite a lot about the subject matter, which in the sample sentence is plumbing. It does not mean: know just a little bit, or know just a small amount.
It can mean know something about if used in the same idiomatic sense.
Jane, please. He does know something about plumbing. Let him do his job.
The phrase might be used in the first person if 1) the speaker wishes to appear modest and not bragging, or 2) if the speaker is frustrated with someone arguing with him/her or keeping him from doing his job.
1) Yes sir, I know a thing or two about the history of Rome.
2) Jason, I do know a thing or two about gardening.
Note: In sentence 1, know does not have an auxilliary verb. In all the other examples, know is emphasized by using do or does.
Each form can be used in 1st, 2nd, or 3rd person, though the modest form is highly unlikely in 2nd person.
Modest 3rd --> Yes Dr. He knows a thing or two about hygiene.
Strong 2nd --> You do know a thing or two about coaching. Two possible interpretations: 1)The speaker realizes that the listener is far more knowledgeable than he/she had thought. 2)The speaker is encouraging the listener. 'You know more than you think you do.' Additional context would determine which interpretation is correct. Orange Blossom