Hitpoints
Hitpoints are your health! They are represented by the bar above your head during any combat.
Your remaining health shows in the green area, how much health you have lost shows in the red area. Select the 'statistics' menu - you will be able to see your 'Hitpoints', you can have an accurate view of your health. This will display the amount of Hitpoints remaining from your total.
You should always keep an eye on this and make sure that the health bar does not reach the bottom, or you will die! To avoid death, you will need to heal during a fight to restore your Hitpoints. For this you will need food and RuneScape Gold. To see what food is 'best' and what heals the most, see the Cooking guide.
If you are losing the battle, you can run away by clicking on the ground. If you do retreat it's best to keep moving, as monsters will pursue you and carry on attacking. If you run far enough away, monsters will eventually give up the chase.