Class to convert roman numeral to integer

Question

could someone help me to understand this python code? I don't understand what does s[i:i+2], roman[s[i:i+2]] and roman[s[i[i]] do.

class Solution():
    def romanToInt(self, s):
        roman = {'I':1,'V':5,'X':10,'L':50,'C':100,'D':500,'M':1000,'IV':4,'IX':9,'XL':40,'XC':90,'CD':400,'CM':900}
        i = 0
        num = 0

        while i < len(s):
            if i + 1 < len(s) and s[i:i+2] in roman: 
                num += roman[s[i:i+2]]
                i += 2
            else:
                num += roman[s[i]]
                i += 1
        return num

s = input('Enter a roman numeral: ').upper()
object = Solution()
print(object.romanToInt(s))