The Tool Rack holds up to two tools that will be made available to your multiblock kitchen. You can also just use it by itself as a ... well, tool rack.
Check the Cooking Table for more information on Multiblock Kitchens.
Right-click the tool rack with an item in your hand to put the item on the tool rack. Right-clicking the tool rack when it already has an item in that spot will retrieve or swap the item out.