A lumpsum investment means putting in a large amount of money into an investment all at once, in a single transaction, rather than spreading it out through smaller periodic instalments like a SIP. This is common when you receive a windfall — a bonus, an inheritance, or proceeds from selling an asset — and want to put it to work right away.
The main advantage of a lumpsum is that your entire amount starts compounding from day one, which can work out better than a SIP if the market happens to rise steadily afterward. The downside is that you're exposed to whatever the market's price level happens to be on that single day, so if the market falls shortly after, your entire investment feels that dip at once. Many investors manage this risk by using an STP to spread a lumpsum into equity gradually instead.