This is tin/lead solder paste, typically used for soldering surface-mount components to a printed circuit board. A syringe or stencil is used to place small amounts of solder on the component pads, then heat is applied to activate the flux and melt the solder. Here's a good tutorial on the process: Solder Paste Stenciling
However, if you're looking for solder paste, the odds are pretty high you already have a good idea how to use it. We're stocking this solder paste because we've been in situations where we'd have liked quick access to bulk solder paste at a reasonable price. This particular brand is available from a popular Chinese retailer, but shipping times often stretch into multiple weeks. Solder paste should be refrigerated as much as possible, so we've already gone to the trouble of express shipping the paste to the USA, where we keep it refrigerated. Shipping within the USA will take only a couple days, which is our experience doesn't affect this paste in any noticeable way.
The paste is 63% tin and 37% lead, for low melting temperature. It's a thick bodied paste that you may want to thin out a little with 99% alcohol before use. The flux is rosin-based and you should clean the PCB with 99% alcohol after soldering.
A 50g jar looks small but contains enough solder for a surprising amount of work. These jars are about 1.5" tall and 1.125" in diameter, and are about 3/4 full of solder paste. We recently constructed 30 PCBs about 4" x 4" and 350 surface mount pads each, and didn't even use a whole jar. For normal hobbyist use, this jar will last a very long time, if you keep it sealed tightly and refrigerated.
This product does contain lead, so you should keep it away from children and food. If you do keep it in a refrigerator that contains food, at least use double ziplock bags to reduce the risk of lead contamination. We recommend a separate mini fridge, or even Peltier-cooled desktop refrigerator.