Often times this product is also referred to as a water heater cover wrap or jacket.
Diy hot water heater blanket.
To make the blanket i took 2 pieces of inexpensive fabric 6 feet of flexible wire 20 feet of carbon heater tape 15mm from www carbonheater us and now on amazon silver conductive wire glue paste female plug 5 5 mm and iron on no sew hem tape.
Very simply as the name implies it is an insulated blanket to cover your water heater.
However in the case of a gas fired hot water heater you will need to be more careful.
For electric heaters turn off the breaker at the electric panel.