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.
The amount that an investor contributes upfront at the start of the fund is known as the initial drawdown, and the subsequent requests for capital are known as "capital calls."