On Stellar, a trustline is an explicit statement on your part that you trust a token. Your account must make a "trustline" to an asset before you can own or trade that asset. Every Stellar account automatically trusts the lumen, and that trustline cannot be revoked.
At the protocol level, trustlines are persistent entries in the Stellar ledger that track the limit for which your account trusts an asset’s issuing account, and the amount of credit from the issuing account that your account currently holds.
StellarX automatically opens a trustline any time you attempt to buy a token, so generally, there’s not need to add them manually or ever think about them, except for one important detail.
- Adds 0.5 XLM to your network reserve
Trustlines persist even after you sell all your tokens. So even after you've sold all of your MOBI, your trustline to it is still open, and thus 0.5 XLM is still being added to your network reserve.
If you zero out a balance, and want to close a trustline, go to “Stellar Network” under “Account Settings.” When you close a trustline, the 0.5 XLM that it added to your network reserve becomes available to you again.