For more in depth information, I'll refer you to this Wikipedia article on digital signatures. But, the long and short of it is that I distribute a public key which would be used alongside that signature to verify if that signature was generated by my private key for the content that is contained in the given post.
content-signature:Hdv6ZJpsd8MxqdThHqSL5gs/cQ+AbxhOPdoRYYOyL8Ip4/dA6VM3oWtTvItLLO1x+I8DiS+Al7ay5e4TasdNNvrXh5cFmq7+b/L523/tJTqheCpv4tNDETp2H6FY9tJa0HmtmIv4jskdeAMrV0Rnmf1HoqMjO729mGdi1fGxLKVIszlBc4TUKtwzLOOFqBYR5zJCeRw7hbNydGnFRCcJcKfhTX/ANkRChqmCU8AR8Vnb99IMUnchWosjno/88WyoVZEpp/M06iMhw63wKsLzwfDySES3UbMAQwLOEYYtC3B8Y+ApeySAfUkssQjVy7bQUtiE7t/5eYoOTCOBQMUJpQ==
I have little to comment on regarding the motivation for your post -- I am not up to date with what's happening in the EU -- but, for an encrypted messaging-app alternative to Signal, I can recommend Matrix.