This field is stored on the item coverage form for an item. It controls the requirement date of the safety stock transaction.
Like you’ve said, today’s date + procurement time moves the safety stock requirement to today’s date + the purchase lead time for an item. This lead time is setup on the default order settings or site-specific order settings forms.
The first issue values moves the requirement date of the safety stock transaction to the same date as the material is first used. This usage might be anything, from a sales order to production order to a sales forecast. In this case, safety stock will not be seen unless there is an issue transaction - AX will not tell you to buy safety stock until you need the material for something else.
I’ve written a blog post that discusses this, as well.