How it works
Base64 represents arbitrary bytes using a limited text alphabet. It is useful when a format or channel accepts text but must carry binary content. Convert.ToBase64String encodes the UTF-8 bytes, and FromBase64String reverses the representation. Anyone with the text can decode it, so this is not encryption and offers no confidentiality or tamper protection. The encoded string is also longer than the original binary data.
Keep those costs and semantics clear in APIs. If a payload needs authenticity or secrecy, use an appropriate cryptographic protocol and key management rather than adding another encoding layer.
Encode a different short word and decode it again. Explain which security properties are still absent even though the encoded text no longer looks like the original word.
A working example
using System;
using System.Linq;
using System.Collections.Generic;
using System.Globalization;
using System.Text;
using System.Text.Json;
using System.Threading;
using System.Threading.Tasks;
var encoded = Convert.ToBase64String(Encoding.UTF8.GetBytes("rune"));
Console.WriteLine(encoded);
Console.WriteLine(Encoding.UTF8.GetString(Convert.FromBase64String(encoded)));
Expected output
cnVuZQ== rune
Encoding changes representation; it does not make content secret.