This is a good question. Simple machines can be any mechanical instrument that gives people an advantage or leverage. to put it another way, simple machines are mechanisms that have a movement, which gives an advantage. This movement can be combined with other movements to create more complex machines. From this perspective, we can say that simple machines are the building blocks of all machinery. Perhaps a few examples would be helpful.
A pulley is a simple machine. It has grooved wheels and allow ropes to be used to lift heavy load. The coordination of the wheels and ropes, make lifting heavier loads much easier. It gives people an advantage over heavy objects.
A lever is a stiff board of some sort, which is on a fulcrum. When weight is placed on one side the other side goes down. Also keep in mind that you can change the fulcrum point, which creates different effects. The common scissor and pliers are based on the principle of a lever.
A wedge is an object with a slant. This can be used to split wood or other things apart. A simple instrument that we have at home based on this principle is a nail.
Other simple machines are: planes and screws.