This quotation is explained by the lines which follow it in the poem. For example, "Its loveliness increases; it will never / Pass into nothingness"; in other words, something which is truly beautiful will only ever become more beautiful, and it cannot die. It "still will keep / A bower quiet for us, and a sleep / Full of sweet dreams, and health, and quiet breathing." A bower refers to a shady, wooded place that feels somewhat enclosed: so, this beautiful thing will, figuratively, remain a private little place for us to enjoy. It will defy "despondence" or "gloomy days," lifting the darkness away from us.
The quotation presents a somewhat unusual statement in that we typically think of things that are very beautiful as things that will not, cannot, last. Youth is beautiful, and it fades. A flower is beautiful, and it dies. Innocence is beautiful, and it is eventually corrupted. Life is beautiful, but we are mortal. For Keats to make such a statement—that things of beauty will never, can never, die—really goes against common "wisdom" and makes this poem unusual.