The yellow horizontal and vertical crosshair determines the current coordinate position in the spectrum.
In the plane (2D view) both a vertical and horizontal yellow line appear. The position of the cursor is given in ppm in the status line at the bottom of the scope.
In the strip only a horizontal line is displayed.
The crosshair is "active" if the corresponding panel is active (pink box displayed at the edges).
An active crosshair can be moved in small steps with the arrow keys and in larger increments with "Shift-Arrow".
Clicking at a new position in the spectrum with the mouse-cursor moves the crosshair to the new position.
Peaks can be selected without changing the crosshair position by moving the mouse cursor onto the peak, holding the shift key and clicking the left mouse button (shift-click). A box appears around the label of the selected peak.
Peaks selected in this way can be moved to the current crosshair position using "move peak". (same thing for moving systems or spins).
Calibrating spectra is done analogously: move the crosshair to the position that the crosspeak should be located, shift-click the crosspeak, and then use "calibrate spectrum".