This is essentially the amount of interest a company pays on debt in a given year expressed as a percentage rate.
Total Interest for the Year / The Total Amount of Debt = Cost of Debt
Why is this important? This is helpful in that it gives you an idea of how much overall risk that has been taken on by the company. Of course it's possible to break down all the numbers and have a much more in depth report, but this, at a glance, gives a quick number on a. is the company healthy (this is obviously just part of that) and b. of course the risk level. When there is so much going on, having one number to look at (that acts almost like an alert) can be very useful.
What to remember: Numbers can drive you crazy and every company has seasons, so look at this in the context of both the season you're in and in the overall financial picture.