That isn't any kind of decent rear shock. All it is is a big spring with a knurled ring you can turn (if it's not seized up already) to either slacken the spring off or tighten it down so it doesn't bounce. If you slack it right off then the 'bob' when pedalling is awful.
No offence intended to the OP but those bikes are advertised as 'full suspension' but they are not (they are just a just a BSO version) and likely if you try and use it seriously off road it will just break. The 'shock' has plastic bushes which wear quickly and the unit will start clonking like mad. They are not serviceable at all. Those bikes also weigh a ton. Ok for using to and from pub/paper shop but otherwise look for a better second-hand proper bike