3 mm Bleed:
The measurements given are exact and 3 mm bleed should be added on each edge to allow for trim. When the artwork is trimmed, slight variations can occur so the 3 mm is there to to avoid a white line being shown on trimmed edges.

The above image is displayed as page one (The front page), page2 would be the reverse side.
The total dimensions including bleed would therefore be:
Width: 127mm (Width page 121mm + 3mm bleed left + 3mm bleed Right)
Hight: 126mm (Hight of page 120mm + 3mm bleed top + 3mm bleed bottom)

2 mm Safe Rule:
No text or detail should be placed closer than 2 mm to trimmed edges or folds, this ensures no loss of detail etc in the folding, trimming process.