Please help me with this code ...i dont understand

The task to do :

Col A Col D

100 4
101 2
102 4
105 2
100 1
101 2
102 3
1055 1

After my code is run, I need for my spreadsheet to look like this

Col A Col D
100 5
101 4
102 7
105 2

And there was one guy post his solution code below :

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Now , based on this code, i have too much confusions:

1: why did he insert column 5?
2, how to understand the COUNTIF systax ?? What is [-1] etc
3,what did he mean for deleting column5, and insert Row (1)??
4,why did he use autofilter?

i am sorry for my confusions, this code is realy awsome, just want to know how does it work,,, because i am a noob now....

Many thanks for any help !!!!!!!!!!!!

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