There are two dangers particular to mobile devices:

1. Losing your bitcoins when you lose, break, or wipe your phone: You must back up your wallet or wallet seed. If you do, then you can always restore your wallet on another phone.

2. Losing your bitcoins through theft of the phone. If your wallet is not encrypted, then somebody can steal your bitcoins by stealing your phone. The wallet must be encrypted and allow access only with a password. If you do that, then nobody can steal your bitcoins.

Do not use a wallet that does not protect against either of these dangers.