Deadheading perennials means to cut off the spent or dead flowers. There are two basic systems that perennial flowers produce blooms and this video discusses/shows them both along with the “how” of pruning perennial flowers.
In short, you can deadhead blooms by taking the entire stem only when the stem goes right to the base of the plant (cut it off as low as possible) No leaves on the stem, take it all the way to the leaves.
Or, when there are leaves below the stem – you cut off the flower and stem and “maybe” a bit of that main stem.
In perennial flowers, this will not overly hurt any plant to have some of the main stems removed during pruning and as you’ll see in the video, I don’t hesitate to cut into the plant when necessary to clean up the stems or spent blossoms.
With annuals, cutting off some of the growing tips is a good idea because this promotes new growth and you need new growth from annuals to produce a lot of blooms.