An anchored token is essentially a credit on the Stellar Network that can be redeemed for the underlying out-of-network asset it represents. Anchored tokens can be thought of as "tethers" to non-Stellar assets, and the ability to easily issue and redeem anchored tokens (aka "tethers") is one of Stellar's most powerful capabilities.
Tokens can be anchored to all kinds of assets including fiat currency, non-XLM cryptocurrencies such as BTC or ETH, bonds, index funds, and real estate investments. On StellarX, you can filter the market view to show particular types of anchored tokens using the navigation tabs at the top.
For more information on anchored tokens, and an explanation of why, for instance, there are 5 different BTC tokens on StellarX, check here.