A mobile wallet is the most common, free and easy-to-use type of self-custody wallet. It is also the least secure. However, it should be noted that this does not mean that it is less secure than leaving your coins on an exchange, but it is less secure than a hardware wallet or multisig wallet.
The reason why it is less secure is because it is hosted on a device (mobile or PC) which has access to the internet. As such, the attack surface for hackers, keystroke observing malware and other nefarious software is increased. A mobile wallet is by default far more secure than an exchange, since there is no counterparty risk taken on by the user when keeping his coins on a mobile wallet, providing he backs up his private keys.
Small amounts (~0.1% net worth) — use a Mobile Bitcoin Wallet
Blue Wallet (Recommended)
This wallet works on both Android and iOS mobile devices as well as a desktop version of Mac’s.
It has a great user interface.
Offers Bitcoin, Lightning and Multisig wallets.
Other recommended mobile wallets
Samourai (Android only)