It's just specialized Unicode characters, you can do all sorts of fun things with them.

https://qaz.wtf/u/convert.cgi

𝕾𝖕𝖍𝖎𝖓𝖝 𝖔𝖋 𝖇𝖑𝖆𝖈𝖐 𝖖𝖚𝖆𝖗𝖙𝖟, 𝖏𝖚𝖉𝖌𝖊 𝖒𝖞 𝖛𝖔𝖜.