Pickling is a method of making food last longer. In pickling, the food is put either into vinegar, or in salt water (called brine). Pickling also changes the flavour of the food. Vegetables are often preserved like this.
The key to pickling is that it produces low pH, 4.6 or lower. This kills most bacteria. Pickling can preserve perishable foods for months. Anti-microbial herbs and spices, such as mustard seed, garlic, cinnamon or cloves, are often added.