There are tooltips that say what you can play when.
But basically, match a color or a numeral to the previous one played. Or you can match +/- 1 on the numerals to try and get straights, like in poker or Yahtzee.
You want to get 3, 4, or 5 of a kind, colors or numerals, or a small straight, like 3,4,5,6, or a large straight, 2,3,4,5,6, or a full house (a pair and a 3 of a kind together). These give you big multipliers. All the multipliers get factored against the number on the card for final values.
Red runes do damage.
Green = healing
Blue = mana
Yellow = block
Purple = weakness curse