Spacing and Kerning»
Each glyph in the font has a bounding box, a rectangle positioned in a theoretical rectangular cell. The most extreme nodes of the glyph determine the bounding box. Each glyph also has sidebearings: extra space to the left of bounding box (left sidebearing, LSB) and to the right (right sidebearing, RSB). The sum of the sidebearings and the bounding box width define the advance width (often just called glyph width).
The process of changing the advance width and sidebearings is called spacing or editing glyph metrics.
The process of changing the space between specific pairs of glyphs is called kerning or editing kerning.
Please see the respective articles: Editing Glyph Metrics and Editing Kerning.