After you have opened GeoGebra, click in the command line at the bottom.
You create the variable a by assigning a numerical value 10 (you are able to change it later) and confirm by pressing return or enter.
In the Algebra Window the object a appears with an empty circular mark;
this means that the object is not visible in the Geometry Window.
Click on the object a and choose Show Object.
The circular mark of object a becomes full to show that the object slider is visible in the Geometry Window.
To create an object that can be modified with the slider, enter in the command line: Segment [(1,2), a] and confirm by pressing Return/Enter.
You have created a segment that begins at point (1,2) with a length determined by the position of the point on the slider. Moving the point on the slider will modified the length of the segment.
If we want to modify the properties of our slider a, click on the object a in the Algebra window and choose the Property, modifying the values and confirm by clicking on Apply.