Farm Dependency is the measure used to describe how reliant a hero is on gold income. Some thrive when they have the money to buy big items, called carry heroes. Others don't require much in terms of items and mainly rely on their spells to be useful. These are called support heroes. There are also a wide range of heroes that fit on the spectrum between the two extremes.
When a hero is very income dependent, it is in the team's best interest to let them farm, gathering more gold income than any other member on the team.
The 1-5 Position System
Farm dependency is a good system to use when making a team composition. It defines five positions that need to be filled by five heroes, assigning them based on farm dependency. A good team composition often requires all five positions to be filled.
Heroes in position 1 have the highest priority for levels and gold on your team, the hard carry. Heroes that fall into this category are usually the weakest on the team before they have an item or two. They often have abilities that let them scale well with bonus stats gained from items, allowing them to deal a ton of damage later in the game. Position 1 heroes are usually placed in the safe lane, with one or more support heroes to babysit them.
Heroes in position 2 have the second most need for gold and levels. A position 2 hero will generally go to mid lane and use their high gold and exp income to control the pace of the game.
The position 3 on your team generally builds toward team fighting items such as Mekansm and Pipe of Insight or initiation items such as Blink Dagger. They are often placed in the off lane where the main goal is to gain is exp and increase levels.
As the farming support on the team, the position 4 will build towards support items that benefit the team such as Urn of Shadows and Force Staff. The position 4 also helps with pure support items such as Observer Wards, Smoke of Deceit, and Dust of Appearance when the position 5 cannot afford to.
The position 5 uses gold on mainly pure support items such as wards, smoke, dust, and sometimes regen and TP scrolls for the other heroes. It is common to see a position 5 hero purchase nothing more than Boots of Speed and a Magic Stick for themselves over the entire game. As a hard support this sacrifice benefits the rest of the team, helping them excel.