If you hold two cards that are the same number in your hand, like two eights or two sixes, you can split them apart and play each one like two separate hands instead of one. Once you split your two cards into two hands, you'll place your original bet with one hand and place an equal bet on the second, split hand.
A key difference between Blackjack and Spanish 21 is that all 10s are removed from the deck in Spanish 21. A Blackjack occurs when the first two cards that are dealt equal 21 a combination of a face card (Jack, Queen, or King) and an Ace (which can have a value of 1 or 11, depending on your hand).