Each symbol corresponds to a number similar to your Vigor level. Notice how the symbol next to the vigor gauge changes depending on how much vigor you have. Theres probably a legend on what symbol is what number somewhere...
Anyway all you really need to know is that you should try to get the same numbers/symbols. Say you roll and you get a pair of 2's and 3 other numbers. Re-roll the 3 other numbers to try and get more 2's.
If you have a pair of 2's and the opponent has a pair of 6's you lose because 6 is greater than 2....but if you had 3 1's you'll win.
Also if you have 5 consecutive digits you have a flush. Insta-win granted you dont re-roll anything. Example: You roll and get 1,2,3,4,5 or 2,3,4,5,6 dont re-roll anything and you'll win unless the opponent has a flush too.
I recommend getting the classic dice set mod @ thewitchernexus.com as the symbols are changed into numbers so you dont have to worry about what the symbols are.
It's like a simplified poker. All sides of the die have a roman numeral in them telling you what number they are. All of the hands not involving face cards or suits are in play. It basically comes down to multiple examples of the same number (2,3,4,5 of a kind), full house (3 of one number, 2 of the other) and a straight progression. Safest bet is just to reroll any dice that you don't already have doubles or better of.