A SIP (Systematic Investment Plan) invests a fixed amount on a fixed date every month. A lump sum invests everything at once. Both buy the same fund; they differ only in timing.
A SIP removes the question of when to invest. Some months you buy at a high price, some at a low, and the average works out. It also matches how most people earn: monthly. A lump sum is right when the money already exists and the horizon is long, because waiting to deploy it has its own cost.
If a lump sum makes you nervous, split it across six to twelve months. The difference in outcome is usually small; the difference in how you feel on a red day is large, and feeling is what decides whether you stay invested.
A SIP is not a return strategy. It is a stay-invested strategy.