When running a reindexing operation for your products, this process starts by requesting the necessary product data to Shopify. Then, Shopify generates a bulk file containing all this product data. This process can take some time (up to 24 hours depending on the size of your catalog). During this time the progress bar will remain at 0% (we still didn't receive any data from Shopify at this point).
Once we receive the bulk file from Shopify, the progress bar starts running to indicate the percentage of data that has been processed on our end.