What the does the line if a = b mean in ruby code? [duplicate]

This question already has an answer here:

• Why does single `=` work in `if` statement? 4 answers

I am trying to understand a particular line in the following piece of code:

def roman_to_integer(roman_string)
   prev = nil
   roman_string.to_s.upcase.split(//).reverse.inject(0) do
    |running_sum, digit|
     if digit_value = DIGITS[digit]
       if prev && prev > digit_value
         running_sum -= digit_value
       else
         running_sum += digit_value
       end
       prev = digit_value
     end
     running_sum
   end
 end

Can someone please help me understand when the line if digit_value = DIGITS[digit] means? are we assigning the value corresponding to the key 'DIGIT' from the hash to the digit_value here?