Most skills have one or more attributes that change the effects of the skill. Not every attribute can appear on any skill - only certain combinations are allowed.
Attributes affect how a skill is written on a card - the format is:
SKILLNAME (N) (S) (to S2) (All) (Y) (X) (every C)
No skill uses all of these attributes at once. Unused attributes are not displayed.
X is the most common attribute. It is an integer, and it controls the magnitude of the skill's effect. For example, Strike 2 does 2 damage, while Strike 3 deals 3 damage. For some defensive skills like Evade and Payback, it instead controls how many times they may activate per round.
Y controls which factions can be targeted by the skill. It is either nothing or the name of a faction. Having no Y attribute means that a skill can target assaults of any faction. For example, Heal 2 can target any assault, even if it is a different faction than the card that used it. If Y is set to a faction, only assaults of that faction or Progenitors can be targeted. Rally Imperial 2 can only target Imperials and Progenitors. At the moment, no skills that specifically target Progenitors exist.
The All attribute allows skills to target all available targets instead of one at random. For example, Strike All 1 hits every enemy assault instead of a single random one. Protect All Imperial 1 will give Protect to all of your Imperials and Progenitors, not just one.
The C attribute limits how often skills can be used. It is often used on skills that would be too powerful if they could be used every turn.
The S attribute is used by skills that affect other skills, and controls which skill is affected by the skill with this attribute.
The S2 attribute is currently only used by Evolve, and indicates what skill the evolved skill becomes.