How to heal in battle

The sign that protects you is Quen, it absorbs part of the damage and prevents you from being incinerated. It is extremely useful, just know that your vigor doesn't regenerate while it's active.

In general, for group battles you should make liberal use of everything you have available. If you have time to prepare, drink a potion (e.g. swallow) and set traps. Use Axii to make one of the enemies fight on your side, distracting the other. Yrden sets up a trap that hurts and immobilizes an enemy for a short while, also very useful. And before you engage in sword combat put on Quen to protect yourself.

But the most important thing is to never get yourself surrounded! Try to seperate the enemies and use hit&run tactics to attack one enemy and quickly evade (double tap direction key or space) once the other enemies arrrive.

The manual has answers to this; I don't know if it's mentioned in game at all.

The Aard Sign - A telekinetic wave that can throw back, knock down or stun an opponent. This Sign can also be used to destroy obstacles, for examplle crumbling walls.

The Yrden Sign - a magical trap placed on the ground, which will wound and immobilize opponents. The Sign's basic level allows you to place a single Yrden Sign on the ground. After upgradin it, you'll be able to place up to three Yrden Signs at a time. These will work in unison, creating an impassable barrier.

The Igni Sign - a gush of flames that wound opponents. When upgraded, the Sign has a chance of incinerating opponents and has a larger area of effect.

The Quen Sign - a protective shield that lasts for 30 seconds at the Sign's basic level. It is a Sign that Geralt uses on himself - the Quen absorbs all damage directed at you, and the upgraded version reflects 50% of the damage at the enemy. While the Quen is in effect, Geralt cannot regenerate vigor.

The Axii Sign - a charm placed on an opponent. If the charm attempt is successful, the enemy will become your ally for a short while, fighting at your side.