Hi medienstadtMünchen,
in English, the menu naming is a bit different than in German, so I guess you are referring to what you enter at Settings > General > Tagline (coming from the backend) or Site Identity > Taglin (coming from the Customizer), respectively.
As far as I understood, you want the tagline to appear bold in the front end of your WordPress website.
That is very much dependent on the theme you are using. Some themes are offering settings to customize the styles.
If that is not the case, you can add custom CSS rules for your WordPress site either somewhere in the theme settings (consult the theme developer documentation for details) or – this is the approach for default themes like Twenty Nineteen etc – go to the Customizer and then click on Additional CSS. Into the CSS field enter the following:
.site-description { font-weight: bold; }

