Mining in RuneScape consists of using a pickaxe on rocks to obtain ores. To be able to mine an ore, you must have your pickaxe in your inventory and then click on a rock (pickaxe types and rock types vary upon mining level).
Once you have mined ore, it can be smelted and smithed into armour or weaponry, or sold for money (gp). This guide will provide you with locations of where to mine and ore and pickaxe tables.