Players.cpp

#include "Players.hpp"

namespace Oasis {
    using namespace eosio;
    using std::string;

    class Players : public contract {
        using contract::contract;
        
        public:
            Players(account_name self):contract(self) {}

            //@abi action
            void add(const account_name account, string& username, uint64_t level) {
                
            }

            //@abi action
            void update(const account_name account, uint64_t level, int64_t healthPoints, int64_t energyPoints) {

            }

            //@abi action
            void getplayer(const account_name account) {
                
            }

        private:

            //@abi table player i64
            struct player {
                uint64_t account_name;
                string username;
                uint64_t level;
                uint64_t health_points = 1000;
                uint64_t energy_points = 1000;

                uint64_t primary_key() const { return account_name; }

                EOSLIB_SERIALIZE(player, (account_name)(username)(level)(health_points)(energy_points))
            };

            typedef multi_index<N(player), player> playerIndex;
    };

    EOSIO_ABI(Players, (add)(update)(getplayer))
}

typedef multi_index<N(player), player> playerIndex;
what is the meaning of this?�

The template is
typedef multi_index<N(table_name), object_template_to_use> multi_index_name;

In C++ typedef creates an alias that can be used anywhere in place of a (possibly complex) type name.

EOSIO_ABI renamed to EOSIO_DISPATCH
https://developers.eos.io/manuals/eosio.cdt/v1.4/index/#eosiolibdispatcherhpp