Stellar is a decentralized platform that aims to connect banks, payment systems and people in one low-cost network. It is powered by a public ledger blockchain and therefore assets can be moved cheaply and efficiently between users. Assets (anything from USD or bitcoin to mobile network credits) get deposited into an anchor or integrator and the asset is then credited to your Stellar account. This credited asset can then be sent across the network and withdrawn by whoever owns the credit. Stellar is essentially a giant transaction layer for any currency or asset, making the transfer of assets between people (especially those in poverty) affordable.
Stellar Lumens (XLM) is the native asset token that is used to pay transaction fees and facilitate trades between two asset pairs if needed.