A blockchain is made up of interconnected blocks, each block stores transactional information.
If we liken this to a book where we write out every financial transaction our business engages in, then the book would be known as the blockchain, and the individual pages would be the blocks.
Now imagine we always give each page the headings: Date, Description, Transaction_Type. As we continue, we might decide to change these headings—for example, adding Cash_Balance—so we now have Date, Description, Transaction_Type, Cash_Balance.
This means future pages will have different headings compared to previous ones. This change doesn't render the old pages null and void, it's simply an improvement.
Soft forks serve the same purpose: they are improvements or changes made to the structure or rules of creating blocks without rendering old blocks useless, unlike with Hard Forks [↗].
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