As you move forward in Rounds, you'll see that certain tiles have the acronyms: DL, TL, DW and TW. These are Multiplier Bonuses that will increase the word's scores:
DL: it doubles the letter's score
TL: it triples the letter's score
DW: it doubles the word's score
TW: it triples the word's score
This means that when you form a word that has any of these marked letters, either you multiply the individual tile value (DL and TL) or the formed word value (DW and TW).