The graveyard is a huge source of advantages for most Commander decks between recursive creatures like Feasting Troll King, spells that return cards from the graveyard to your hand like Seasons Past, creatures that bring things back from the graveyard like Sun Titan, and commanders that relentlessly leverage card advantage from the graveyard like Muldrotha, the Gravetide or Meren of Clan Nel Toth
"Kicker [cost]" means "You may pay an additional [cost] as you cast this spell." If a spell's kicker cost was paid, the spell is "kicked." The kicker ability doesn't let you pay a kicker cost more than once
These explanations are not exhaustive and are only intended to serve as a high-level overview of the experiences each card creates in Commander
At the beginning of each end step, if a card left your graveyard this turn, Gau deals damage equal to its power to each opponent

Explorers of the Deep EDH Precon Commander (1) Creature (37) Benthic Biomancer Coralhelm Commander Deeproot Elite Kiora's Follower Master of the Pearl Trident Merfolk Mistbinder Merfolk Skydiver Metallic Mimic Nicanzil, Current Conductor Stonybrook Banneret Thieving Skydiver Cold-Eyed Selkie Emperor Mihail II Evolution Sage Kopala, Warden of Waves Kumena, Tyrant of Orazca Merfolk Cave-Diver Merfolk Sovereign Merrow Reejerey Realmwalker Sage of Fables Singer of Swift Rivers Svyelun of Sea and Sky Thassa, God of the Sea Topography Tracker Vorel of the Hull Clade Deeproot Historian Herald of Secret Streams Seafloor Oracle Surgespanner Tributary Instructor Zegana, Utopian Speaker Mist Dancer Tatyova, Benthic Druid Prime Speaker Zegana Xolatoyac, the Smiling Flood Tishana, Voice of Thunder Instant (9) Rapid Hybridization Growth Spiral Ripples of Potential Beast Within Inspiring Call Quandrix Command Aetherize Ruinous Intrusion Commit / Memory Sorcery (6) Bygone Marvels Curse of the Swine Explore Kodama's Reach Ravenform Wave Goodbye Enchantment (6) Hardened Scales Simic Ascendancy Branching Evolution Deeproot Waters Kindred Discovery Reflections of Littjara Artifact (5) Sol Ring Arcane Signet Simic Signet Swiftfoot Boots Commander's Sphere Land (36) Alchemist's Refuge Command Tower Forest x7 Hinterland Harbor Island x13 Karn's Bastion Llanowar Reborn Mosswort Bridge Myriad Landscape Path of Ancestry Reliquary Tower Rogue's Passage Secluded Courtyard Simic Growth Chamber Temple of Mystery Temple of the False God Unclaimed Territory Vineglimmer Snarl Strengths and Weaknesses Explorers of the Deep, helmed by Hakbal of the Surging Soul, is a merfolk typal deck that focuses on flooding the board with merfolk, then exploring every turn for card advantage and to buff your critters
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PSA Graded Ultra Rare and Secret rare cards sounds great right