What does “ StringBuilders are not thread-safe” mean?

Question

I have read some articles about the pros and cons of String and StringBuilder in the Java Programming language. In one of the articles, the author

Answer 1

StringBuilder inside a method would be safe.

public void addProperty(String name, String value) {
    private StringBuilder sb = new StringBuilder("1=2");

        if (value != null && value.length() > 0) {
            if (sb.length() > 0) {
                sb.append(',');
        }
        sb.append(name).append('=').append(value);
    }
}