What is the meaning of "__attribute__((packed, aligned(4))) "

It is C language.It is written that:

typedef struct __attribute__((packed, aligned(4))) Ball {
    float2 delta;
    float2 position;
    //float3 color;
    float size;
    //int arcID;
    //float arcStr;
} Ball_t;
Ball_t *balls;

Please tell me what is the meaning of it,and how to use this keyword.