Normal: If you have a PC who can defend by using Reactions form a Squad with another character, assisting the PC’s actions, the PC loses the ability to perform Reactions.
Skirmisher Talent: If you have a PC who “buddies up” with another character, the PC still retains the ability to perform Reactions.
Individual PCs (and all Toughened and Nemesis type NPCs) can react to attacks or other events by generating Doom and performing a Reaction - which turns the opponent’s attack into a Struggle, an opposed type of action.
Example: NPC A, the attacker, slashes with his sword at PC B. As B is declared target of A’s attack, the player of B may declare to take a Defend Reaction against this attack. So A will roll Agility+Melee usually against a Difficulty 1. And the defending B will roll Coordination+Parry usually against a Difficulty 1. Both sides compare the amount of Momentum generated by the attack and the defense. Whoever has more Momentum wins and gets to keep the difference in Momentum to use for Momentum spends or to put into the Group Momentum pool.
If in the example above B would have been with character C in a Squad, B would normally not been able to perform a Reaction. So if attacker A succeeded in the Agility+Melee test, A would deal damage to B.
Squads are a special case of groups, having only a single turn, despite being made up of a number of characters. The advantage is, that on the offensive the other characters perform an Assist to the action of the leader of the Squad. The disadvantage is that a Squad just like a Mob of Minion NPCs does not get any Reactions.
The Skirmisher Talent allows for the special case of a character in a Squad with just one other character (as a kind of “buddy” team) to still retain the ability to perform Reactions. That makes a huge difference.