Magnesium firestarters have a magnesium block attached to a small ferro rod strip that together can make sparks. Scrape the magnesium off with a knife or the provided striker, and then build up a small pile of the shavings.
This is what you want to strike into. Once the spark catches, the magnesium will burn hot, fast, and bright.
Magnesium doesn’t work alone as tinder to start a fire. You still need to find dry tinder to place the shavings inside. Magnesium burns around 4,000 degrees F, which helps get a fire started more quickly, but it can take a lot of work to get enough shavings from the magnesium block.