Roshi literally means "old teacher". It is a Japanese title of respect for a senior Zen teacher, placed after the name, as in Suzuki Roshi.
How the title is used depends on the school. In Rinzai Zen it is reserved for a teacher who has completed the full course of koan training and received formal authorization. In Soto Zen it has been used more loosely as a mark of respect for an older priest.
In Western Zen centers, usage varies from one community to the next.