What do these dashed-box symbols mean on Japanese keyboard? What do these dashed boxes mean and how to use them?
Your image seems to have disappeared so I added one myself.

The dashed boxes represent the space that a normal character would occupy so you can see how the characters you're typing will be positioned.  On the middle row shown, the first key puts a mark in the lower-left position, the second key has the same position, the third shows that the dot will be in the middle of the character space and the last one shows the line will be beneath the normal character space, almost like an underscore