SIP is a method of scheduling investments (the other being Lumpsum).
When investing via SIPs you make payments over a regular period in the fund of your choice.
For Eg. An investor who wants to invest ₹60,000 with a Mutual Fund scheme over a year, can do so via Lumpsum (by investing the entire amount at one go) or start a SIP by investing ₹5,000 every month.
SIPs are considered an easy way to start investing, as they allow you to benefit from factors such as Rupee Cost Averaging (Investing across market highs & lows to bring down overall cost) & can be started for amounts as low as ₹100.