In a chord diagram, the highlighted part that represents the number 1 usually indicates the nut.
In guitar chord diagrams:
- The nut is where the strings rest at the guitar headstock and is often depicted as the line at the top of the diagram.
- Open strings are typically represented by circles above the corresponding lines.
- Chord symbols usually appear above or below the diagram.
- Left-hand fingers are indicated by numbers placed on the fret lines.
Therefore, in the context of your question, the highlighted part containing the number 1 represents the nut.