Complex sentence: Attacking creatures that weren't blocked deal damage equal to their power to the player (s) or planeswalker (s) they attacked; the amount of damage dealt is deducted from the player's life total or the planeswalker's loyalty counters.
Split into simpler sentences: Attacking creatures that weren't blocked deal damage equal to their power to the player (s) or planeswalker (s) they are attacking. The amount of damage dealt is deducted from the player's life total or the planeswalker's loyalty counters.