It means "practice" or "practice (you all)". It's used to give a command or instruction to a group of people to practice something.
Here's a breakdown:
* Practicar: To practice
* Practiquen: Practice (you all)
Examples:
* Practiquen el español. (Practice Spanish.)
* Practiquen el piano. (Practice the piano.)
* Practiquen las reglas. (Practice the rules.)