What are Lumens (XLM)?

Stellar has a native built-in token called the lumen, created to fill a unique role in the network. By design, Stellar requires that each account hold a small number of lumens at all times, helping ensure the network's overall safety and security.

Having lumens in reserve activates a wallet on the network, allows it to send transactions, create trustlines to other Stellar assets, and pay network fees.

Without Lumens, the Stellar network ledger could become overloaded due to no restrictions on account creation or submissions to the ledger, which could disrupt and overload the network.

For more info: https://www.stellar.org/lumens