The rebuild cost is to completely rebuild an identical house to yours, including the cost of labour and materials.
Typically, the cost to rebuild your home is less than the market value, which you might find in your Structural/Building Survey or Home Buyers Report under 'Rebuild Value'. It's important to estimate the correct amount, so you don't pay too much for your insurance or be underinsured if you need to claim. Remember to include garages and outbuildings and consider any special features or materials.
The ABI offers a rebuild cost calculator to help you, or you can get an accurate calculation through a surveyor. You can find a local surveyor by contacting Royal Institute of Chartered Surveyors (RICS).