General meaning: "together" or "jointly"
* cooperate: to work together
* coexist: to live together in harmony
* coauthor: to write something together
* coworker: a person who works with another
* coeducational: having both boys and girls
* cofounder: a person who starts something with others
* co-pilot: a pilot who assists the main pilot
* co-star: an actor who shares a starring role
* co-sign: to sign a document along with someone else
* cohabit: to live together as a couple
* co-parent: to share parenting responsibilities
* co-owner: someone who owns something with another person
Other meanings:
* cogency: the quality of being clear and persuasive
* cognizant: aware of something
* cohort: a group of people who share a common characteristic
* collage: a work of art made from different materials glued together
* collateral: property pledged as security for a loan
* combustible: capable of burning
* commemorate: to remember and celebrate
* commence: to begin
* concur: to agree
* congregate: to gather together
* conspire: to plot together secretly
* coordinate: to make things work together harmoniously
This is just a small sample. There are many more words with the prefix "co-". If you have a specific context in mind, please let me know and I can give you more relevant examples.