The Book of Ruth is a short book of four chapters. In our Bibles, it is placed after the Book of Judges because it takes place in the same period (1:1).
the redeemer
A key theme in the book of Ruth is that of the redeemer (Hebrew: goel). What this entails is already described earlier in the Tanakh, where we find laws regarding the right of redemption (e.g., in Lev. 25:23-55). Israelite law stipulated that the closest blood relative had the right to repurchase lost family property. This relative is called the redeemer.
Boaz
The book of Ruth illustrates what redemption entails and puts it into practice. Boaz, who plays a key role in this book, is the most famous redeemer in the Old Testament. He is a representation of Christ, the Redeemer.