This is the 2347th most frequent Russian word.
народный
"Народный" means "folk," "popular," or "national" in English.
The word "народный" here is used as a title to refer to an artist recognized by the public or state as having exceptional contributions, often used formally.
Народный артист исполнил песню.
The people's artist performed a song.
The word "народной" describes something belonging to or characteristic of the people, often reflecting traditional or communal aspects.
Эта песня считается народной.
This song is considered traditional.
In this usage, "народный" qualifies the museum as one focused on the culture, tradition, or history of the people.