The basic six expressions (size, center, warning time, warning distance, damage buffer, and damage amount) have equivalent commands that are explained at
Minecraft Wiki. The
effect resets those six to their default values, which are also shown in Minecraft Wiki above.
The syntaxes to do with border stabilizing have to do with whether the border is currently moving. When the border is not moving, it is considered "stabilized".
code_language.skript:
time remaining until border stabilize in %world%
is just telling how much time is left before the border is stabilized.
code_language.skript:
on border stabilize [in %world%]
is an event that triggers when the border becomes stabilized in a world.
Not sure if this is more clear, if you still have some confusion you can ask about specific syntaxes, here is a list for reference:
https://www.skunity.com/search?search=border