In RuneScape, when you pick up or purchase an item like armor, materials, RuneScape gold, it will be added to your inventory, into the backpack. Backpack just like the store in the game that takes part in important part.
In the game window backpack icon, your inventory is represented. Take the Left-click to access your inventory. In Inventory interface, the backpack section of your inventory displays all of the items that you are carrying at the time.
You should notice that large items will take up one inventory slot each. Small items such as RuneScape money can all be stored in just one slot, in which case a number at the top left of the slot indicates how many of that item is currently grouped together. Just because that consider with the room with slot, it is good way to exchange or buy RuneScape gold.

How to inventory
Move the mouse pointer over the item which you want to interact in your inventory. And perform the default action associated with the item. Most of the time the default action will be the use command. Once you have completed these steps, you can click on the target item in your inventory or in the game window. The right check will give the option to view other action you could perform with an item. Then the action you can display will be in a menu, the default one is in the first. Once the menu is displayed, you can then select other commands such as Drop, Examine etc.

Drop Rules
When your slot is full you could choose to drop some items, then they will appear at your feet. On condition that the item's market value is 3,000 RunScape gold or less, other players would see it and pick it up after one minute. You may only leave drops for other players to pick up with a total value of 3,000 RS Gold or less in any fifteen-minute period, any other items you drop will not become visible to other players. It is the very important rules when you exchange with items with other players.
Note that a stack of items worth more than 3,000 RS gold will never appear for other players. The amount you drop that becomes visible to other players is constantly recalculated, but will never exceed 3,000 runescape gold.