Your "available to trade" amount represents how much you have available to buy stocks or ETFs. It includes unused cash from your deposits as well as any proceeds from stocks or ETFs you've sold.
Your "available to withdraw" amount represents how much settled (and cleared) cash is in your account and can be withdrawn.
Sometimes the amount you can withdraw will be different from the amount you have available to trade. Typically this would be because you have made recent trades that haven't settled yet! Trades take 2 business days to settle. In addition, new deposits cannot be withdrawn until they meet the mandatory 5 business day clearing period as per Anti-Money Laundering regulations.