Field Advantage also refered as Field Control follows the same basic principles as Hand Advantage, but with a few key differences and variations.
Field Advantage usually does, but not always mean, how many monsters you have on the field, compared to your opponent. Protection Spell and Trap Cards such as "Starlight Road" are also needed to determine this as there are quite a few Mass Monster Removal cards.
Field Advantage also extends to when a player has more dominance than the other due to his cards giving him a key advantage. Examples include Legendary Six Samurai - Shi En with his ability to negate a Spell or Trap during either player's turn those reinforcing the offense and defense of the Six Samurai. Evilswarm Ophion that blocks Special Summon of high-level monsters while being a strong monster itself and can search cards for protection or more offensive. Evolzar Dolkka and Evolzar Laggia that can make most plays difficult to perform.