Milestones are a way to divide the fixed-price job into a series of deliverables based on your client's needs. They can break a larger contract into more manageable chunks. For more information about milestones for Fixed-Price jobs please check out this help article.
To ensure that you are payment protected on Upwork, we always recommend that you only submit work for the fixed-price contract through the contract page by clicking on the "Request for payment" button. As you already requested the payment, your client will have 14-days to review the work that you have submitted. If they don't come back within 14-days, the money in escrow will automatically be paid to your account. You can check this help article for more information.