Clarify the terms

[24seconds]

          I guess maybe you are not the only one who misunderstands the term. Let me follow up clarifying the terms. 🙇

Originally posted by @24seconds in #185 (comment)

[24seconds]

There are some terms and concepts used in rust-cli-pomodoro app but it's not clear. Therefore, it leads misunderstanding. Clarify the terms and resolve this problem.