Simply put, a mana shield is the 'physical hardening' of one's aura to protect from simple mana and physical attacks. Mana shield is a basic defensive spell, allowing one to project one's aura out like a shell, typically an arms-length ahead of themselves. Someone who specializes and is skilled in mana can project it to even 2 or 3 arms length, providing cover for a person close by.
The mana shield is most effective at reflecting or softening the blow of mana-based attacks, but lacks true solidity to prevent strong physical attacks. It is an imperfect defense, and is used often in times of desperation.