A systematic investment plan (SIP) is an arrangement in which an investor puts a fixed amount into a mutual fund at regular intervals, usually monthly, often by automatic deduction from a bank account. The name is widely used in India and Pakistan; elsewhere it is often called an automatic investment plan. The method behind it is dollar cost averaging (DCA), which is explained in full, with a worked example, on that page.
Because the same amount buys more units when prices are low, a SIP spreads purchases over time and supports a long-term investment strategy.