Instruction box for other users

Hi,
I don't know if it is possible to do what I want but I will ask anyway.

On my spreadsheet I have a column with dropdown boxes. Users can select Tier 1,2,3 or N/A for that column. What I want is an instruction box that tells them what those Tiers represent so that they know which one to pick. Ideally I would like that explanation box to appear when they hover the mouse over cells in that column.

If further explanation is needed, please ask!

Thanks in advance