As in Systematic Investment Plan where one invests a fixed amount of money in a fixed interval, say every month, in a mutual fund—in case of a SWP, an investor would withdraw a fixed amount of money every month from a mutual fund. SWP is useful when one wishes to create an income stream from their investments, which many retired people are taking advantage of.
You first need to select the fund, then decide the rate at which you want to withdraw the amount, you may also decide a fixed amount or can also ask for appreciation amount only. As mutual funds are not a fixed return instrument and also the withdrawal rate is as per your choice, so the total return from the structure depends on product/Mutual fund return and your withdrawal per month.