What is "declare" in Bash?

After reading ilkkachu's answer to this question I learned on the existence of the declare (with argument -n) shell built in. help declare brings: > Set variable values and attributes. > > Declare variables and give them attributes. If no NAMEs are given, > display the attributes and values of all variables. > -n ... make NAME a reference to the variable named by its value I ask for a general explanation with an example regarding declare because I don't understand the man. I know what is a variable and expanding it but I still miss the man on declare (variable attribute?). Maybe you'd like to explain this based on the code by ilkkachu in the answer: #!/bin/bash function read_and_verify { read -p "Please enter value for '$1': " tmp1 read -p "Please repeat the value to verify: " tmp2 if [ "$tmp1" != "$tmp2" ]; then echo "Values unmatched. Please try again."; return 2 else declare -n ref="$1" ref=$tmp1 fi }