Skill Specialization Slots are unlocked as your character levels up, to a maximum of 5 (to match how many skills can be equipped on the Action Bar).
Upon unlocking a Skill Specialization Slot, you can choose to Specialize in a skill that your character currently knows. Doing so allows you to level up that skill and invest points into its Specialization Tree. Skills have a natural maximum level of 20.
If you would like to specialize in a different skill, you can press the Respec button at the top right of the Skills Window while viewing the tree for a specialized skill.
Newly specialized skills will be automatically set to your Minimum Skill Level, and may receive a bonus to XP so they can catch up to your other skills.
You can see your current Minimum Skill Level on the Respec button inside a skill's tree. Skills currently affected by a XP bonus will state so above their XP bar on the left side of their tree.