According to the official Stellar lumens documentation (available here):
Before an account can hold an asset another account issues, it has to establish something called a trustline. Trustlines are an explicit opt-in for an account to hold and trade a particular asset.
A trustline is an explicit opt-in to hold a particular token, so it specifies both the asset code and issuer. Each trustline increases an account’s minimum lumen balance by one base reserve — currently 0.5 XLM — and tracks the balance of the asset the account holds. Trustlines can also limit the amount of an asset an account can hold, though more often than not, account holders don’t set that limit: they simply turn the trustline on, which by default allows the maximum.
Note: to enable a trustline your wallet must be active and have a minimum balance of 0.5 XLM.
If you don't know how to activate your wallet look here.
In short, each trustline will require and reserve 0.5 XLM which is returned upon removal of the trustline.
There are 2 methods to add a trustline: