The Chip Quik No-Clean Liquid Flux Pen is great for all rework, solder, de-solder, and reflow applications! In particular this one is from the rework-champs at Chip Quik.
Liquid flux helps solder to "cling" to metal by removing oxide buildup. It also helps spread out solder by reducing surface tension. Liquid flux is crucial for surface mount soldering and desoldering, but usually not necessary for through-hole components. With a pen, you can just 'draw' it onto the area you're working. We also have a liquid tacky flux if you prefer using a syringe.
Flux is a cleaning fluid, so use care and ventilation when in use!