When you invest lump-sum, there is always a risk that you put money when the market was high. If the market goes down in the short term thereafter, you lose some capital.
In an SIP, you put in a fixed amount every month. In a particular month, if the market was high, you earn less units of the fund for your money. If the market was low, you earn more units. That way, you end up buying more units when things are cheap and less when they are expensive. Thus, you are shielded from short term fluctuations of the market and benefit from its growth in the longer term.