Balancer Pool Tokens (BPT) are ERC20 tokens that represent your share of a specific pool. A wallet holding BPT is entitled to a share of the pool, so if you lose your BPT, you lose access to your liquidity. BPT are created (“minted”) when liquidity is added, and they are destroyed (“burned”) when liquidity is removed.
The best way to think of owning BPT is as though you own a percentage of that pool, similar to how owning shares of a company works
Some projects build on top of Balancer pools and require you to stake your BPT tokens to earn their rewards. Please note that by staking BPT you are effectively giving your tokens to that contract - you will no longer receive BAL rewards as it goes to that contract you have staked in instead!