Need explanation about forloop code in python

i found below python "for loop" online and its syntax differs from other codes , i like to know about :

1. why the 'key' is in front of for loop
2. why there is a if statement inline

CODE:

input = "codeforcode"

freqDict = Counter(input)

freq1 = [ key for (key,count) in freqDict.items() if count==1]

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further i like to have web links to understand "for loop" more depth in python.