def purify(x): #You are declaring a function called purify and saying that it needs a value import which will be saved as x.
number =  #You are creating a variable called number and the variable type is list and the list is empty.
for i in x: #you are creating a for loop that is going to loop through the list (x) by how many index values(i) there are
if i % 2 == 0 #You are checking to see if the index value the loop is currently on is completely divisible by 2
numbers.append(i) #You are saying that if i is divisible by 2 then add or append it to the numbers list
return numbers #After the loops completes its iteration and all the values are added, it will send the value of numbers back to where the function was called.
print purify([1,2,3,4]) #You are calling the function to take the numbers in parentheses and process them in the manner that I just explained and after processing, take the value that was returned and print it.
Hope this helps