First of all for the earth's gravity to be 1/2 of what it is today, the earth's mass needs to be 1/2 of what it is today because gravity depends on the mass of an object. That's why the moon has a much smaller gravity that the earth because it is much smaller.
Second if the earth's gravity is 1/2 of what it is today, the weight of all things will be reduced by half, in other words you will be making half the force on the floor, you are making now. This means we feel lighter to walk and jump.
Third if you drop an object in the air (free fall) it will go down with half the speed it normally falls (normally it falls with an acelleration of 9.8m/s2)