With this add-on you can easily mirror the weight paint data of bones and their corresponding vertex groups to the other side. This is for example a great help, when you have a symmetrical model and you forgot to paint your weights with the x-Mirror-Option. With this add-on you can project your working status to the pose bone on the other half of your object.
Your character has to be skinned to the armature first. When you than make changes to the weight paint of your model, you can copy that to the bones on the other side. You will find the installed add-on in the weight paint mode of your object. This works with one or more bones.