Encryption using modular addition and a key Problem i'm facing says:
 The value representing each row is encrypted using modular addition
with a modulus of 32 and a key of 27.

I sort of figured out what modular addition is for myself an hour ago but the key thing confuses me.
What does this 

key of 27

mean? And how do I find out the original value in the very end?
Text of the task

The image in Figure 1 is to be encoded and encrypted using the
  following steps. 1 Each of the five horizontal rows of the picture is
  encoded as a 5-bit binary number. White squares are encoded as 0s and
  black squares as 1s. The top row is therefore encoded as 00010. 2
  Starting from the left, the first digit is multiplied by 16, the
  second by 8, the third by 4, the fourth by 2, and the fifth by 1. For
  the first row the result is (0 × 16) + (0 × 8) + (0 × 4) + (1 × 2) +
  (0 × 1)
  = 0 + 0 + 0 + 2 + 0
  = 2. 3 The value representing each row is encrypted using modular addition with a modulus of 32 and a key of 27. 4 The result is
  converted to a 5-bit binary number. 5 The binary number corresponding
  to one row of the original picture is then used to fill a
  corresponding row in a blank grid. 1s are displayed as black squares
  and 0s are displayed as white squares. Choose from the options below
  the one pattern that represents the correctly encoded and encrypted
  image.

Thanks in advance xx
 A: Consider alphabet of 32 letters:

'  ' = 0;
'a' = 1;
'b' = 2;
'c' = 3;
. . .
'v' = 22;
'w' = 23;
'x' = 24;
'y' = 25;
'z' = 26;
 and other symbols - up to 31:
'+' = 27;
'-' = 28;
'(' = 29;
')' = 30;
'=' = 31.
Now, encrypt the text $-$ is to add value 27 to each symbol value:
Let plain text is "a+b=c".
Digital interpretation: (1,27,2,31,3).
Encryption:
$1+27 \equiv 28 \mod 32$;
$27+27 \equiv 22 \mod 32$;
$2+27 \equiv 29 \mod 32$;
$31+27 \equiv 26 \mod 32$;
$3+27 \equiv 30 \mod 32$;
So, cipher text is $(28,22,29,26,30)$. Of, in letter format: "-v(z)".

Edit.
If you need to encrypt black/white images like 
$\Box\Box\Box\blacksquare\Box \;,\; \Box\blacksquare\Box\blacksquare\blacksquare \;,\;$ ...
then same way:
1-st symbol: $00010_2 = 2_{10}$; $\quad$ $2+27\equiv 29 \mod 32$; $\quad$ encrypted : $29_{10}=11101_2$; 
2-nd symbol: $01011_2 = 11_{10}$; $\quad$ $11+27\equiv 6 \mod 32$; $\quad$ encrypted : $6_{10}=00110_2$; 
So, encrypted image will look like this:
$\blacksquare\blacksquare\blacksquare\Box\blacksquare \;,\; \Box\Box\blacksquare\blacksquare\Box \;,\;$...

Finally, to decrypt, it is necessary to subtract $27$, or (the same) to add $5$ (because $32-27=5$).
