The Loose Death tab shows you if any of the deaths that are inexact in your tree could be "tightened up" by taking account of any facts you know about the person being alive.
An example of how this works will explain the thinking. Let's say a man is known to be dead in 1891 as his wife is widow on 1891 census. He was born in 1833 married in 1856 and had 5 children, born in Q2 1858, Q3 1861, Q4 1864, Q2 1867 and Q1 1869. He was alive on the 1861 census.
So what can you tell about his death. Well you know he died BEF 1891 and you know he was alive in 1861 as he was on the census so you might have a death of BET 1861 AND 1891. However you also know he must have been alive
at least 1868 to have fathered a child in Q1 1869. So if your death entry only said BET 1861 AND 1891 then you would have a "loose death" and it would suggest "tightening" it to BET 1868 AND 1891.