Simplifying geometries of polygon shape files in QGIS's SimpliPy

First you have to install SimpliPy

Note that the plug-in says: You must have installed Shapely 1.2.18 or you will get an error installing or using this plug-in. Shapely is open source.

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Here you can learn what simplifying geometries of shape files is.
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SimpliPy was successfully installed and has been added to the menu.

Just in case, if you have never ever opened a shape file in QGIS, you can do this like here:

(and then search for your file on the disk).

 SimpliPy window

Select layer with geometries you want to simplify

Features are in this case administrative districts of Sweden and I want to simplify all of them.

Parameters
Smaller parameter = more vertices (map consists more details) and changes compared to the basic map are very small => final map will respond slowly.
Bigger parameter = less vertices (map consists less details) => final map will respond quickly. More you will find here: Simplifying geometries of polygon shape files.

Output
Mark Add result to canvas (visible layer)

Prevent shape removal
If you leave it blank all small shapes that are small islands will disappear after simplifications with high parameter, as they are not separate administrative districts. If you need all the islands, just place a tick.

Tick Repair intersections.

Results with different parameter.

Other simplifying option is built in QGIS

Simplify tolerance works like parameter in SimpliPy.

add tick to Add result to canvas so you'll see your new simplified layer in QGIS's layers list.

I'm not a geographer, I need shape files for data visualizations so I search for easy to use tools that don't force me to work too hard. I've noticed that SimpliPy produces maps without overlaps and gaps. When I use that standard QGIS built in option I get some problems with gaps.