The kimchi jar should be lightly covered during fermentation, not sealed tight, to allow the carbon dioxide gas produced to escape and prevent the jar from exploding.
Kimchi fermentation is an anaerobic process, meaning it occurs without oxygen. During this process, the bacteria present in the kimchi consume sugars and other compounds, producing lactic acid, which gives kimchi its characteristic sour flavor, and carbon dioxide gas. If the jar is sealed tightly, the accumulating carbon dioxide will build up pressure inside the jar. This pressure can become significant enough to crack or even explode the jar, creating a mess and potentially being dangerous. Therefore, it's crucial to allow the gas to escape. The best way to do this is to use a fermentation lid with an airlock, or simply loosely cover the jar with a lid or plastic wrap secured with a rubber band. This allows the gas to escape while still preventing unwanted contaminants from entering the kimchi. Burping the jar daily, by briefly opening it to release the pressure, is also a common practice if you are using a lid that is not specifically designed for fermentation.
When packing your kimchi into the jar, leave about an inch or two of headspace at the top. This provides room for the kimchi to expand as it ferments and helps prevent overflow when the gas is released.