RS Puppis is a Cepheid variable star. Thanks to the work of Henrietta Leavitt at Harvard, we know there is a relationship between the period of variability and the luminosity (intrinsic brightness) of a Cepheid variable. This means we can use Cepheid as a standard candle to as one way to determine distance to the really far out star or galaxy the stars reside in.
Distances to closer objects can be determined by other means like the parallax but that tends to be less accurate at great distances. One of the reasons the ESA Gaia launch is so exciting is that parallax measurements will be very accurate and to greater distances than we can do currently. Distances to the stars are very difficult to get accurately.
From NASA (links to a full res image and it is amazing!) with acknowledgment to H. Bond (STScI and Pennsylvania State University):
This festive NASA Hubble Space Telescope image resembles a holiday wreath made of sparkling lights. The bright southern hemisphere star RS Puppis, at the center of the image, is swaddled in a gossamer cocoon of reflective dust illuminated by the glittering star. The super star is ten times more massive than our sun and 200 times larger.