Warrior skill level is the level at which you perform all Warrior skills. All players have a base warrior skill level equal to whatever level warrior they are. This can be increased or decreased with equipment having an effect of WARRIOR_SKILL_LEVEL by X.
To view your current warrior skill level, type show skill. You will get a message such as:
Your skill level is level 33 thief, level 35 warrior.
Which is your current Warrior skill level with any bonuses from equipment or spell affects added.