A show cause letter is a written notice asking the employee
to explain or to "show cause" why he should not be given disciplinary
action given what he did or did not do.
As for whether a reminder is necessary prior to the show
cause letter, it really depends on the labor laws of the place where you work
in. In some jurisdictions, the show cause letter itself serves as the first
notice. The employee then submits his written explanation and then a second
notice is required called a Notice of Decision.