Any investor with any money at all is going to come up against the same question at some point. Should you invest it all at once or spread it out over time? There are very good arguments on both sides and the correct answer is going to depend on the maths and on how you personally manage risk and emotion.
Lump sum investing is where you put all of your available cash into the market at once. Dollar cost averaging (often abbreviated to DCA) is the process of splitting that sum into equal parts and investing them at regular intervals (e.g. weekly or monthly). So if you have $12,000, the lump sum approach is to put all $12,000 in today, whereas DCA would see you invest $1,000 per month over the course of a year.
The case for the lump sum is essentially a case about time in the market. Over the long term the share market goes up more often than it goes down. That means the money that is invested earlier has more time to grow than the money that is sitting in cash. There's a whole host of studies out there (including some very well known Vanguard studies) which have demonstrated that lump sum investing has outperformed DCA in around two thirds of the historical periods that they've looked at. The reason for that is pretty straightforward really. If the market goes up more often than it goes down then waiting generally means you're buying at higher prices later on.
The case for DCA is really all about managing your regret and your risk. If you invest everything and the market drops 20% next month, that's going to hurt and it could well be the trigger for you to sell at the very worst possible time. With DCA you mitigate that risk of bad timing on a single large entry. A drop in the market simply means your next purchase will be at a lower price and you'll have bought more shares. It doesn't in any way guarantee you'll do better, it just reduces the risk of bad timing on a single large entry which is why so many people find it much easier to just go with it.
It's also worth bearing in mind that most people are already doing DCA without even realising it. If you're putting a portion of each of your paychecks into a pension or a brokerage account then you're effectively doing DCA. You don't have a large sum of cash to deploy so the choice is made for you. In that situation the sensible thing to do is just to keep on investing and not to try and time it.
Fees and account rules matter as well. If every purchase is subject to a commission or minimum then breaking your money up into a lot of small purchases could end up costing you more. Most of the major brokers these days will do commission free stock and ETF trading so that's not as much of an issue as it used to be, though it's still worth checking. Taxes can also be an issue in a taxable account so it's worth knowing how your gains will be taxed.
What about swing traders? The principle doesn't really apply as swing trading is about specific setups with defined entries and exits rather than long term buying. The mindset does however carry over to your long term money. A lot of traders will have a separate long term account where they are just steadily investing into broad index funds and a smaller trading account for the active setups. That separation of accounts is what prevents their trading mistakes from impacting their long term plan.
Which is better? Mathematically lump sum wins more often. In practice the best strategy is the one you're actually going to follow. If you know that a sudden drop would make you panic and sell then DCA may be the thing that protects you from yourself. If you're comfortable with the swings and you've got a long time horizon then lump sum has the advantage on average. A middle ground would be to invest some of it now and then spread the balance over a few months.
This is general education not personal financial advice. Your timeline, your objectives and your comfort with risk are what should be the drivers of the decision and a qualified financial professional can help you apply it to your own circumstances.