New To This Need Some Help To Understand This Cover

About 5 years ago a bought a small stamp album.I started researching and my god mother saw my interest so she gave me my aunts collection.Her collection had some single stamps blocks and whole sheets mint as well as mini mint collections from the 20 s to 90.Sge then collected first day covers .She moved on to collect the souvenir sheets till she passed .I want to learn about the covers and so forth as it imterests me.I am finding it difficult to understand what makes it worth more and so forth .zglad to be able to see peoples input makes it easier