Here is a short article covering the laymen's terms of Enjin, Efinity, why Efinity built on Polkadot, and Jumpnet.
Basically, Enjin is an NFT coin for users, gamers, and developers to use to mint their digital gaming assets so that they will have a worth of Enjin's market value. Here's the link to further read about Enjin: https://enjin.io/about/why-enjin
Whereas; Efinity is another blockchain in the crypto world that is mainly to provide a highway for the NFT craze at a minimal cost. The purpose of Efinity's token (EFI) is to use as payment for users to utilize the blockchain.
As for Polkadot; let's take the example of a cabinet. Now each drawer represents a different blockchain. In reality, you can simply transfer a file from a drawer to another; but in the crypto world, it is impossible because each blockchain is designed differently. Thus, Polkadot was born to solve the issue. To simply put, it acts as a bridge for users from different blockchains to interact with each other without having issues about the difference each blockchain is built, governed, and secured. Here's another article that also writes about the laymen's term of Polkadot: https://www.ledger.com/academy/crypto/polkadot-for-dummies
Now that we solely understand Polkadot's ecosystem, then we could also understand why Efinity builds on the Polkadot ecosystem.
Enjin, simply, wants to provide a highway for the NFT craze that can interact with different blockchains. In order for Enjin to achieve such a goal, the more logical thing to do is to create Efinity on the Polkadot ecosystem.
Now with Efinity, users can transfer their NFTs to various blockchains at a minimal cost.
For Jumpnet; it is another highway for users to transfer their Enjin coin and other ERC-1155 coins on the Ethereum ecosystem for free.