SIR, or Systematic Investment Plan, is a method of investing in mutual funds. It allows investors to contribute a fixed amount of money at regular intervals, such as monthly. This approach helps in averaging the cost of investment over time and can reduce the impact of market volatility. Investors can start a SIR with a small amount, making it accessible for many. By investing consistently, individuals can build wealth over the long term, benefiting from the power of compounding returns. SIR is a popular choice for those looking to save for goals like retirement or education.