NMX is a decentralized token that can be bought, sold, farmed or added to the liquidity pool with USDT.
NMX-LP is a liquidity token that can not be bought and sold anywhere. This liquidity token can be obtained only by placing NMX and USDT in the same proportion in the NMX/USDT liquidity pool.
To put it simply, NMX-LP is like a contract in a bank which proves that your assets are in the liquidity pool. You opened a deposit in the bank, you received a receipt. To get your deposit back you need to show this receipt.
After that, you can only do 2 actions with NMX-LP:
Place it in staking for additional earnings in NMX tokens;
Return it to the pool to return the invested funds. After that, the smart contract burns NMX-LP tokens.