Fifteen stone. Hurled into. Dung.
A piece from Animal Farm.

Without halting for an instant, Snowball flung his fifteen stone against Jones’ legs. Jones was hurled into a pile of dung and his gun flew out of his hands. But the most terrifying spectacle of all was Boxer, rearing up on his hind legs and striking out with his great iron-shod hoofs like a stallion.

Snowball is a pig. I don't understand what he did.

Question

Fifteen stone. Hurled into. Dung.
A piece from Animal Farm.

Without halting for an instant, Snowball flung his fifteen stone against Jones’ legs. Jones was hurled into a pile of dung and his gun flew out of his hands. But the most terrifying spectacle of all was Boxer, rearing up on his hind legs and striking out with his great iron-shod hoofs like a stallion.

Snowball is a pig. I don't understand what he did.

Answer

A stone is a British measurement of weight - 14 pounds or 6.3 kilos. 15 stone would be 210 pounds, 95 kilos (about) . That is what the pig weighed. 
"Snowball flung his (weight)..." 
Hurled into = threw into 
dung = manure. 
Snowball hit Jones with his full body. The force of the hit threw Jones into the manure pile.

Answer

“Stone” is a British unit of measurement.  “Fifteen stone” is about 210 pounds or 95 kilos.  In other words, Snowball flung (or pushed) himself against our friend Jones’ legs. Ouch!