Dollar cost averaging means investing the same amount every month, whatever happens. When the market falls, that amount buys more units; when it rises, fewer. The average price paid ends up below the average of the prices.
Its main virtue, though, is not mathematical but behavioral: it turns a hard, repeated decision (is this a good moment to buy?) into an automatic transfer that requires no decision at all. It removes the part of the process where most money is lost.
One honest caveat: if you already hold a large cash sum, history says investing it at once usually beats spreading it out, simply because markets rise more often than they fall. Spreading it out is worse on average and far better if the realistic alternative was never investing.