#!/bin/sh
echo "What is your name?"
read USER_NAME
echo "Hello $USER_NAME"
echo "I will create you a file called $USER_NAME_file"
touch $USER_NAME_file

Think about what result you would expect. For example, if you enter "steve" as your USER_NAME, should the script create steve_file? Actually, no. This will cause an error unless there is a variable called USER_NAME_file. The shell does not know where the variable ends and the rest starts. How can we define this?  The answer is, that we enclose the variable itself in curly brackets:

#!/bin/sh
echo "What is your name?"
read USER_NAME
echo "Hello $USER_NAME"
echo "I will create you a file called ${USER_NAME}_file"
touch "${USER_NAME}_file"