Primaris Astartes are superior versions of the Space Marines project. As such, their genes, conditionning and physical abilities are greater than those of normal Astartes. This grant them the following abilities.
Super Soldiers: In exchange of a fate point, at any moment during a turn, a Primaris Space Marine can make a full action (or two half actions) at the initiative level the controlling player wishes. This also means that a Space Marine can make two turns during one. So the Space Marine could make a full action to move and make a lightining attack and then, during his normal initiative order, still do an attack action, as these counts as two seperate turns for purpose of actions. On the other hand, this sepcial use of a fate points do not grant an other reaction.
Superior Geneseed: By the superior quality of their making, the Primaris Space Marines can reroll one test of their choosing per combat or narrative action. They must accept the second result even if it was worse than the first.
And They Shall know no fear: Be it by their dogmatic confidence in the Imperium's superiority and their role in defending it or the warped minds of chaos astartes that have been polluted by the horrors of the Warp, Space Marines are rarely impaired with fear. After suffering any mental effect (influence by psychic power, fear, mental trauma, madness, etc.), a Space Marine may test at the beginning of each following turn his willpower to Snap Out of It.