Checkpoints are hardcoded into the Litecoin Core client. With checkpoints set, all transactions are valid up to the checkpoint condition valid. This is set in case anyone wants to fork the blockchain and start from the very same block; the checkpoint will render false and won't accept any further transactions. The checkpoints.cpp file helps in managing the checkpoints in a blockchain source code:

namespace Checkpoints {

    CBlockIndex* GetLastCheckpoint(const CCheckpointData& data)
    {
        const MapCheckpoints& checkpoints = data.mapCheckpoints;

        for (const MapCheckpoints::value_type& i : reverse_iterate(checkpoints))
        {
            const uint256& hash = i.second;
            BlockMap::const_iterator t = mapBlockIndex.find(hash);
            if (t != mapBlockIndex.end())
                return t->second;
        }
        return nullptr;
   }