It's a staple in Korean cuisine, dating back to an amazing 37 BC when fermented foods were widely available. It is used on the side of most dishes to lift the flavour and add a little bit of spice. It comes with an all-star nutritional scorecard and some 1.85 million metric tones is consumed in South Korea, nearly 40 kg per person!
We are talking about Kimchi. 
Spicy fermented cabbage. Slightly sour, spicy and umami flavour. I add it to everything!
It is made with different vegetables and ways depending on the season, but most commonly, it would be made with the napa cabbage or Wong bok, which are sweeter and more tender than the European type cabbage heads.
I buy my Kimchi from my local asian supermarket, but I'm about to start making my own which I'm excited about. Once you are introduced to Kimchi you can really see why the Koreans eat so much of it.
Here's a recipe that's quite simple.  

Homemade Kimchi
Prep time: 30 - 45 minutes
Makes 1 ltr
1 medium head Wong Bok
½ cup iodine-free salt
water
1 tbsp ginger, grated
1 tbsp garlic, grated
1 tsp sugar
3 tbsp fish sauce
1 to 5 tablespoon Korean pepper flakes, available from Asian supermarket
2 cups cucumber, peeled and cut into matchsticks
4 white onions, peeled and sliced thin
Cut the cabbage lengthwise through the stem into 1/4. Cut the cores out and then dice the cabbage into 4 cm squares.
Place the cabbage in a large bowl and sprinkle with the salt. Using your hands, massage the salt into the cabbage until it starts to soften a bit.
Add enough water to cover the cabbage. Place a plate on top to weigh down the cabbage and leave to stand for 2 hours.
Rinse the cabbage under cold water a couple of times and set aside to drain.
Add the rest of the spice ingredients.
Gently squeeze any remaining water from the cabbage and add in the spice paste, cucumber and onions. Work the paste through all the vegetables.
Pack into a jar, pressing down until the liquid rises above the cabbage. Seal and leave in cool space out of direct sunlight for 1-5 days depending how ripe you would like it.
Open the jar every day and press the cabbage down to release any trapped gases.
Transfer jar to the fridge to keep for up to two months.