An insulator is something that prevent or slows the transfer of heat?

Pretty much, yes. Each material has a property for how easy it is for heat to transfer into it. If you have a bowl of hot soup and dip a metal fork into it and then a wooden fork, the metal fork will come out hotter because heat flows into metal more easily. An insulator is something that doesn't take heat very well. If you surround something with something that doesn't take heat, there's nowhere for the heat to go and the thing inside stays warm.