The thrush is cracking a snail on an 'anvil' stone.
The hard shell is broken by thwacking it again and again, and then the soft succulent flesh is gobbled up.
Each thrush is likely to have its own particular stone, or stones, for snail-cracking; sometimes quite small ones, but at other times large ones.
The art of snail-cracking is not an instinctive one.
Each young thrush has to learn by watching what other thrushes are doing, and then copying them - but the young thrush does not find this easy at first.