In Path of Exile, nearly all action RPGs had linear skill systems where the progression of an active skill was preordained by the developers. If you were a fire user, you would progress from a weak fire spout to a more powerful fireball, and eventually to something big and flashy like a flame wave.
Throughout Path of Exile, you'll acquire many poe items with colored sockets that correspond to the game's three attributes of Strength, Dexterity and Intelligence. You can socket the skill gems you find into these colored sockets to immediately gain the use of that particular skill. Each skill gem is aligned with a single attribute. For example, the Fireball skill is aligned with Intelligence, and therefore appears on a blue gem.
The color of sockets on items is strongly biased towards the attributes that the item is aligned with. For example, a Strength/Dexterity sword is very likely to contain red or green sockets and unlikely to have blue sockets. Skill gems can only be placed in a correctly colored socket.
Skill gems can be removed from sockets by right clicking on them. There is currently no penalty for doing this. We want players to experiment with the nearly infinite combinations our skill system allows.
Items can have multiple sockets, which can be different colors. Though the early items you encounter will contain one or maybe two sockets, the number will increase throughout the game. Most armor pieces can have up to four sockets. High end body armor can reach six sockets. One-hand weapons and shields cap out at three sockets, while two-hand weapons can contain six.
The above is an explanation about the sockets. Meanwhile, we can't ignore the fact that poe orbs is important in this game, you can buy chaos orbs on U4GM.