What does the magazine volume and issue numbers mean?

Volume = year, issue = sequence

Typically publications use volume to indicate the number of the year of the publication's life. That is, the first year of publication is Volume 1, next year is Volume 2, and so on. Within volume numbers, the issue indicates its sequence. The first issue of a particular volume is 1, the second is 2, until you reach the last issue in that publication's year. It might be Issue 365, it might be Issue 12, it might be Issue 4 depending on how often it's published.

Volume numbers do not necessarily coincide with calendar years. A publication which starts in the middle of the year would publish Volume 1 until it begins its second year. So Volume 2 might start in May, or August, or whenever.

The reason they don't use just issue numbers is that a publication might be quite old, and simple issue numbers would grow quite large. A 100-year-old daily newspaper, which is not rare, would be in issue 36,500 if it were to use only an issue number. Shorter-run publications like comic books can get by with only issue numbers. Which just might indicate the publisher doesn't plan for the series to last all that long.