The Rosegarden matrix editor enables you to view and edit music in a simple, logical format: each note represented as a block in a grid, at a height corresponding to the pitch.
The matrix view contains Grid and Quantize menus in its main toolbar.
The Grid menu controls the size of the visible grid in the background of the matrix view, as well as the time resolution used when entering and moving notes. After you set it to a particular time value, the normal note insertion and drag operations will then snap automatically to grid points spaced at that time width. You can override the snap-to-grid effect temporarily by holding down Shift during editing, or you can disable it by setting the Grid menu to None.
The Quantize menu works rather differently. Rather than controlling the behaviour of other editing tools, it actually performs an action. When you change the value in this menu, the selected notes -- or the whole segment if nothing is selected -- will immediately be quantized using a grid quantizer of the chosen resolution.
The Quantize menu also shows you continuously what resolution the current segment or selection is quantized to. For example, if you select a set of notes that start on consecutive quarter-note beats, the Quantize menu will update itself to show 1/4, as this is the coarsest resolution consistent with the existing quantization of the current selection.