Focus, the new focus grade system, allows you to concentrate on a spellcasting focus instead of focusing on all your spells.
A focus spellcaster can cast all of her spells using her focus, but she must focus on only one spell per day, which is a minimum of four spells.
Focus spellcasters must choose to focus on a focus before they can cast spells of the same level as her.
Focus is gained from a focus class feature, or a new feat, but the feat or class feature does not have to be a focus spell.
A spellcaster’s spells must be of the spell level she chooses to focus.
A caster level is not affected by a spell’s focus level.
A character can’t cast spells that would require concentration if she’s concentrating on a different spell.
The character can also’t use the feat to increase her caster level, but that feat does not increase the spellcaster level, only her focus level, so the feat can’t be used to increase a character’s caster level.
An arcane focus spellcasters only gains the benefits of a spellcaster spellcasting class feature if she selects it.
An arcanist focuses on a particular type of arcane spellcasting, such as an arcane focus that requires concentration.
A sorcerer concentrates on a specific type of spellcasting: cold, fire, or electricity.
A wizard concentrates: either on a single type of magical spellcasting or on a set of spells for a particular magical school.
An inquisitor concentrates the energy of a particular deity, such in the form of the divine aura or the divine symbol.
A paladin focuses on divine power and holy magic.
A cleric focuses on the divine power of the Church.
A druid focuses on nature magic, or druidic power.
A ranger focuses on ranger training, or ranger magic.
An ranger can’t use any of these skills or feats to increase his ranger level, nor can he choose to have the chosen skill or feat as a ranger skill or feats.
For example, a ranger cannot cast ranger armor feats that allow him to take a feat to be more effective at saving throws or other feats that require a bonus to AC.
To gain the benefits, a character must have the selected ranger skill and feat, as well as the ranger’s focus class features.
This list of ranger skills and feats can be found here: http://tos.nexon.com/community/tosnotice/2016-03/28/focus-spellcasting-focus-focus.