What you witnessed could potentially be a phenomenon known as a "flare" or "outburst" from a star. Flares occur when there is a sudden and temporary increase in the brightness of a star, followed by a rapid decrease. Flares are often associated with certain types of stars, such as young, active stars with strong magnetic fields.
These flares are caused by energetic processes within the star, typically involving the release of magnetic energy. The sudden increase in brightness is a result of the intense release of energy, which can cause the star to temporarily become many times brighter than its normal luminosity. As the flare subsides, the star's brightness returns to its usual level, causing it to disappear from view.
Another possibility is that you observed a satellite or space debris reflecting sunlight. Satellites in low-Earth orbit can sometimes catch sunlight and appear as a rapidly moving "star" that brightens and then fades as it moves into or out of the Earth's shadow.
To determine the exact cause of the phenomenon you witnessed, it would be helpful to note the location and time of observation. This information, along with further details about the event, could assist in identifying the specific object or phenomena you observed.