Transformations
The Transformations filter is split into three dialogs: Transform Metrics, Transformations, and Interpolate with Background.
Transform Metrics
Glyph → Transform Metrics assigns either a new width or new sidebearings to the selected glyph layers.

Enable Width to assign a new width to all selected glyphs. Check on both sides to modify both sides equally to match the new width. Otherwise, the points on the layer keep their original coordinates.
Uncheck Width to set new sidebearings. Control whether the left sidebearing (LSB), the right sidebearing (RSB), or both should be changed using the checkboxes next to their fields. If Relative is selected, the LSB and RSB values are added to the current sidebearings (or subtracted if the entered values are negative). Otherwise, the LSB and RSB values overwrite the existing sidebearings.
Transformations
Path → Transformations applies a linear transformation to the selected points or selected layers.

Translate moves points by a given X and Y value. Origin defines the point from which the transformation originates. Scale scales the selection along the horizontal and vertical axis. If the lock button is locked , the horizontal value is used for both axes. Slant skews the selection without optical correction. Click the Slant label and choose Cursivy for an optically corrected skew. Note that Cursivy requires horizontal and vertical metrics to be set in File → Font Info… → Masters → Stems.
Interpolate with Background
Path → Interpolate with Background results in an interpolation of the foreground and background layers.

A value of 0 % leaves the foreground unchanged; 100 % replaces the foreground with the background, and values in-between result in an interpolated shape. Use a value below 0 % or above 100 % to extrapolate. Note that this filter does not work if the foreground and background are not compatible.