Spell Speed is a way of telling if a card can be chained on another card or not. Simply put, it's the "Speed" of the card in question. There's a simple way to identify a cards Spell Speed:
Normal Spells, Ignition Monster Effects: Spell Speed 1
Normal Traps, Quick-Play Spells, Quick Monster Efects: Spell Speed 2
Counter Traps: Spell Speed 3
Keep in mind, that cards with equal or higher Spell Speed can chain, but cards with lower cannot.
E.G.:
Chain Link 1: Dark Hole
Chain Link 2: Solemn Judgment
Chain Link 3: Seven Tools of the Bandit
This is a possible chain, since it goes from a SS1 to SS3 and followed by another SS3.
Chain Link 1: Negate Attack
Chain Link 2: Mystical Space Typhoon
This chain is impossible, because a SS2 card is trying to chain on a SS3 card, which it can't.
Subject: Spell Speed 
