Your BRD wallet connects directly to the Bitcoin network. This improves security and privacy, but also means your wallet can get out of sync with the network on a few occasions. The BRD app has a Fastsync feature that syncs your wallet almost instantly.
Manually syncing with the blockchain can take a lot of time, and usually isn't necessary. If someone sent you bitcoin and the transaction is confirmed on the blockchain, but it is not appearing in your wallet, your wallet may be out of sync. First check to make sure the address the bitcoin was sent to is correct.
If you are trying to send bitcoin and constantly get an error message that your transaction was rejected, it could also mean your wallet is out of sync.
When Fastsync does not work for you, using this feature will rescan the blockchain to find your bitcoins and display it properly in your wallet. Visit this page for instructions on how to manually sync your wallet to the blockchain.