It can't be over-emphasized how important it is for you to use TimeTracker and always insert a memo for each period of TimeTracker's tracking. Under hourly contracts, Upwork apparently pays you for your hours for each Upwork work week and then later gets reimbursed by your client.
If your client doesn't pay Upwork (through outright fraud, not replacing an expired credit card,etc.), Upwork will review all of the time you have booked on TimeTracker and charge back to you the amount they have paid you for poorly or undocumented usage of TimeTracker in order to reduce their loss on your client's non-payment.
As you get more familiar with Upwork's practices and requirements, you may decide to use manual hours - it apparently works well for some freelancers. However, my standing advice to all new freelancers is to avoid using manual time if at all possible. A client never has any obligation to pay you for manual time you bill them with TimeTracker, even if they have agreed to "allow" you to book it.