Details

Possible settings are:
dist the same margins on all sides

{{left,right},{bottom,top}} different margins on different sides
Margin sizes are in printer's points.

LegendMargins adds space between the content generated by LegendLayout and the container provided by LegendFunction.

Examples

Leave no margins between legends and LegendFunction:

Wolfram Language code: SwatchLegend[63, Range[5], LegendFunction -> (Framed[#, FrameMargins -> 0]&), LegendMargins -> 0]

Specify margins in different sides:

Wolfram Language code: LineLegend[63, Range[5], LegendMargins -> {{5, 10}, {15, 20}}, LegendFunction -> (Framed[#, FrameMargins -> 0]&)]

Leave no margins between legends and plot:

Wolfram Language code:

ContourPlot[Sin[x  + Cos[y]], {x, 0, π}, {y, 0, π}, PlotLegends -> Placed[BarLegend[Automatic, LegendMargins -> 0], {{1, 0.5}, {0, 0.5}}]]

Use LegendMargins to leave margins between legends and plot:

Wolfram Language code:

ContourPlot[Sin[x  + Cos[y]], {x, 0, π}, {y, 0, π}, PlotLegends -> Placed[BarLegend[Automatic, LegendMargins -> 15], {{1, 0.5}, {0, 0.5}}]]

Top

	dist	the same margins on all sides
	{{left,right},{bottom,top}}	different margins on different sides