I means one. 
V means five. 
X means ten. 
L means fifty 
C means one hundred
D means five hundred
M means one thousand 

then if the position of the LOWER OR EQUAL numeral is on the right, it will be added. 


VI means six. 
Five plus one is six. 

or if 

XX means twenty 
ten plus ten is twenty 

but if the position of lower numeral ONLY is on the left, it will be subtracted from the higher numeral on its right. 


IX is nine. 
one is lower than ten so it will mean ten minus one. 

For more complicated form: 


means five hundred plus one hundred but then,  
XX is twenty but placed before L which is fifty. so fifty minus twenty is thirty. and one is smaller than five so five minus one is four.

500+100+30+4 = 634

so this is how you know an answer in roman numeral form.