str is a function that turns its arguments into strings.

user=> (str 123)
"123"
user=> (str :a-keyword)
":a-keyword"
user=> (str false)
"false"
user=> (map str (range 10))
("0" "1" "2" "3" "4" "5" "6" "7" "8" "9")

When str is given multiple arguments, then it will concatenate them all into one big string.

user=> (str "I need " 5 " of these")
"I need 5 of these"
user=> (str "In "
#_=>        (rand-nth ["summer" "winter"])
#_=>        " I like to go "
#_=>        (rand-nth ["swimming" "running"]))
"In summer I like to go swimming"

References