This is where you put forward a price that you would like to buy the property for, to the estate agent acting on behalf of the seller. Their estate agent will put your offer to the seller and the property becomes ‘Under offer’.
If your offer is rejected you could make another, higher offer if your budget allows.
Once your offer is accepted then you can proceed with the purchase and application for a mortgage. The property is usually taken off the market at this point and the property becomes ‘Sold subject to contract.