This iExpression links an arbitrary property to a slider (or some other 1D property - like audio keyframes). The link is such that the slider controls a time remapping of the property. You specify which frame corresponds to the minimum value of the slider and which one to its maximum value. For all other slider values the iExpression chooses automatically an appropriate frame (using linear interpolation).
Link to Slider
Don't put a number here. Use the link button to link this to a slider control, an audio amplitude or some other 1D property.
the smallest value of the slider control (or of the audio level or whatever other property you link to)
the largest value of the slider control (or of the audio level or whatever other property you link to)
Frame for Min
the number of the frame whose value the property should have when the slider has its minimal value
Frame for Max
the number of the frame whose value the property should have when the slider has its maximal value
If this option is enabled, the property will always take values from the range of frames from 'Frame for Min' to 'Frame for Max', even if the slider it is linked to has values smaller than 'Slider Min' or larger than 'Slider Max'.