Notes identified by x's usually indicate the lyrics should be spoken.
X's on an accompaniment part usually indicate clapping or a similar rhythm. They are also called "dead" or "ghost" notes, indicating there isn't an actual pitch to be played, but rather are percussive.
A sheet music preview showing X's instead of lyrics may not be appropriate for children, most commonly due to profanity. A sample preview may show words spelled as 'Xxxxx', but the lyrics will not be X'd out on the purchased product.