A Withdrawal is the opposite of a Deposit.
When you input a Withdrawal Transaction, the value of the corresponding Investment decreases. So a Withdrawal occurs when for example, you sell some shares and move the cash you have generated to your bank account.
On Exirio, we use the term Sell instead of Withdrawal every time you are working on Securities, Cryptocurrencies or Precious Metal: but the meaning is exactly the same.
On the graphs, changes in value caused by a Withdrawal (or Sell) are marked with a W (or S).