"Leading" is vertical spacing added between lines of type. See e.g.
http://en.wikipedia.org/wiki/Leading
In Word I think you would probably do this by setting
Format|Paragraph|Indents and spacing|Spacing|Line spacing to "Exactly" and
putting 10pt in the "At" box. But that assumes that what you have been told
is what is actually required. They might want, for example /at least/ that
amount of vertical spacing (for that you could use "At least" instead of
"Exactly". Probably best to check back before getting too committed.