what exactly does it mean when people say "a mishna"?

Shalom, Thank you for your question. The Mishna is the complied work of the Rabbis from the Ta’anic period – around the time of the Second Temple. “A Mishna” is one teaching from the entire work – which spans six tractates, divided into over 60 separate volumes. Each volume (a “mesechet”) is further divided into chapters, and the chapters into individual Mishna’s. The Mishna is the concise statement of the Oral Torah. The Talmud (Gemara) is the later commentary on the Mishna. I hope this is of some help. (You could also try looking up the word “Mishna” on the internet). Blessings.