don't understand example about buntdb HOT 1 OPEN

import (
	"fmt"
	"time"

	"github.com/tidwall/buntdb"
)

var BUNT *buntdb.DB

func Bunt() *buntdb.DB {
	client, err := buntdb.Open(":memory:")
	if err != nil {
		return nil
	}
	fmt.Println("BuntDB connect")
	return client
}

type Store struct{}

// set key to db
func (s Store) SetToBuntWithExpireTime(key string, value string, expire int) (err error) {
	timer := time.Duration(expire) * time.Second
	err = BUNT.Update(func(tx *buntdb.Tx) error {
		tx.Set(key, value, &buntdb.SetOptions{Expires: true, TTL: timer})
		return nil
	})
	return err
}

// get key from db
func (s Store) GetFromBunt(key string) (value string, err error) {
	var val string
	err = BUNT.View(func(tx *buntdb.Tx) error {
		val, err = tx.Get(key)
		if err != nil {
			return err
		}
		return nil
	})
	return val, err
}

// set key to db
func (s Store) SetToBunt(key string, value string) (err error) {
	err = BUNT.Update(func(tx *buntdb.Tx) error {
		_, _, err := tx.Set(key, value, nil)
		return err
	})
	return err
}

// delete key from db
func (s Store) DeleteFromBunt(key string) (err error) {
	err = BUNT.Update(func(tx *buntdb.Tx) error {
		tx.Delete(key)
		return nil
	})
	return err
}
func main() {
	BUNT = Bunt()
	store := Store{}
	err := store.SetToBuntWithExpireTime("key", "value", 3600)
	if err != nil {
		panic(err)
	}
	val, err := store.GetFromBunt("key")
	if err != nil {
		fmt.Println("no key find")
	}
	fmt.Println(val)
}
`