Fund leads can choose whether they want to require all capital from investors when the fund starts, or if investors can break the capital up into more than one transfer. When a lead chooses to have investors contribute capital at multiple points, and they want to request the next installment from investors, they "call" capital.
If you've invested in a fund with capital calls, your lead should provide you with an idea of when capital calls will take place. When they initiate a capital call, you'll receive an email from AngelList with the amount to transfer and the deadline. If you have questions around the deadline or amount, you can discuss with your fund lead directly.