So, I'm struggling trying to understand this kinda simple exercise

def a(n):
    for i in range(n):
        for j in range(n):
            if i == 0 or i == n-1 or j == 0 or j == n-1:
                print('*',end='')
            else:
                print(' ',end='')
        print()

which prints an empty square. I tought I could use the code

            print("*", ''*(n-2),"*")

to print the units in between the upper and the lower side of the square but they won't be aligned to the upper/lower side ones, which doesn't happen if you run the first code... so... could this be because of end='' or print() (would you be so kind and tell me what do they mean?)?

Check the reference page of print. By default there is a newline character appended to the item being printed (end='\n'), and end='' is used to make it printed on the same line.

And print() prints an empty newline, which is necessary to keep on printing on the next line.

EDITED: added an example.
Actually you could also use this:

def a(n):
    print('*' * n)
    for i in range(n - 2):
        print('*' + ' ' * (n - 2) + '*')
    if n > 1:
        print('*' * n) 

In Python 3.x, the end=' ' is used to place a space after the displayed string instead of a newline.

please refer this for a further explanation.