en_tn_condensed/mat/21/44.md

Whoever falls on this stone will be broken to pieces

Here, "this stone" is the same stone as in Matthew 21:42. This is a metaphor that means the Christ will destroy anyone who rebels against him. This can be stated in active form. Alternate translation: "This stone will break into pieces anyone who falls on it" (See: rc://en/ta/man/translate/figs-metaphor and rc://en/ta/man/translate/figs-activepassive)

But anyone on whom it falls will be crushed

This means basically the same thing as the previous sentence. It is a metaphor that means the Christ will have the final judgment and will destroy everyone who rebels against him. (See: rc://en/ta/man/translate/figs-parallelism and rc://en/ta/man/translate/figs-metaphor)