What do "Marginal" and "Good" mean when connected to an ADS-B receiver?

The signal quality status displayed in the top-left corner of ForeFlight's Maps view when connected to an ADS-B receiver is an indicator of FIS-B (weather) reception from ground-based ADS-B towers. It does not indicate the reception quality of traffic, either air to air or from ground-based ADS-B towers.

Signal Quality

Meaning

No Towers

No FIS-B weather available.

Marginal

Some weather data will be available but there may be gaps or CONUS radar may not be available.

Good

Weather data should be complete and CONUS radar should be available if broadcast while Good reception is indicated.