DCA is short for Dollar Cost Averaging. It is a strategy that has gained popularity when it comes to buying Bitcoin but whose principle is quite simple.
You have a fixed amount of fiat currency with which you want to buy Bitcoin. Instead of buying Bitcoin all at once, you break it up into smaller amounts and space out your purchases over a period of time.
For example, you have $1,000. You divide that into 10 x $100 smaller and buy $100 worth of Bitcoin every week for 10 weeks.
This buying method is supposed to even out the volatility of the Bitcoin price so that you get an average. There are pros and cons to a lump sum, so it depends on your risk aversion.
Normally, people wait for the price to appreciate and then sell for a profit.
DCA is not the same as buying Bitcoin daily, because with DCA you have a fixed amount with the intention to sell. Buying Bitcoin daily is like saving Bitcoin, so the mindset is a bit different between the two.