A lump sum invests everything at once. A SIP spreads the same money over many months.
If you already have a large amount sitting idle and a long horizon, a lump sum can put more money to work earlier. The downside is that you are fully exposed to whatever the market does in the weeks after you invest.
A SIP reduces the risk of investing everything at a temporary high and is easier on monthly cash flow. The trade-off is that some of your money stays in cash longer and therefore has less time to compound.
Many people use both: keep a core SIP running, and add lump-sum amounts when they receive bonuses or when valuations look reasonable after a discussion with their adviser.