Ticking the export to web box is the most important part and signifies when the system should consider that product to be created. When you check this it looks for the source group attached to the sales channel and if that is been set that then decides what stock levels are shown.
If these two are both enabled it then considers the manufacturer sku with any other product assigned that same manufacturer sku. It will combine them together and use the sizes and colours that you have set to work out the drop-down do need to apply ain WooCommerce. If these are not unique or if you have forgotten to allocate a size or colour to one of the variations then the product will be held off until it is complete.
Once these are created they will be in a draft state allowing you to add images and pictures and confirm that they are correct before publishing.
The stock levels will be updated for products even if they are in a draft state.
If you have installed other plug-ins they may not allow the product to be created unless the description is unique and other fields are established as required by that plug-in.
The description used will be that of the lowest product ID in their grouping.