To limit excess network activity on Stellar, accounts must hold a minimum balance of 1 XLM before they are established on the blockchain. Each additional asset held in that account also requires a reserve fee of 0.5 XLM to establish a trustline. This trustline is an explicit, pre-approved connection between the issuer and the token holder.
The initial account activation and each additional asset trustline are established using an on-chain transaction that requires a signature and fees. The transaction appears in the recent activity panel and transaction history.
For example, if there are asset wallets for ten different confirmed Stellar tokens in a vault account, your XLM wallet must maintain a minimum balance of six XLM: one for the base asset, and 0.5 XLM times ten tokens.
Learn more about funding a new Stellar wallet.
For more information, see the official Stellar documentation regarding minimum balances and asset structure.