A lumpsum invests everything on day one. If the market rises steadily from there, a lumpsum wins outright, since all your money was earning returns the whole time. But if the market falls right after you invest, a lumpsum takes the full hit immediately.
A SIP spreads the same money across many purchase dates. If the market falls in the early months, your later instalments buy more units at lower prices — this is rupee-cost averaging, and it cushions the damage compared to a lumpsum that went in right before the drop.
In practice, most long-term equity market history shows a slight edge to lumpsum investing over very long horizons, simply because markets rise more often than they fall. But SIPs win on something lumpsum can't offer: they don't require you to have a large sum sitting around, and they remove the temptation (or anxiety) of trying to time when to invest it.
A practical middle ground many people use: invest a lumpsum gradually over 6-12 months using an STP (Systematic Transfer Plan) from a liquid fund into equity, rather than either going all-in on day one or trickling it in through a slow monthly SIP.