I recommend that you be explicit about all elements of the plot. Explain how the boxplot indicates the median (mean?), quartiles (quantiles?), and extreme values (distant quantiles?)... assuming that's what you're plotting. I suggest you be explicit here not only for clarity, but also because the general boxplot template can be used to display different statistics together. The wikipedia article discusses some variations.
You will rarely be criticized for being too explicit, but it's easy to criticize for being too ambiguous. As a rule of thumb, make as few assumptions about your readers as possible.