Adventure cards are two spells attached to a single card where the adventure spell may be cast from the hand and then the card is placed in exile
Whenever you attack with one or more Creatures, you can add that much mana using any combination of Red and Green, and that mana doesnt empty from your pool until end of turn
Creatures (27) 4 Mother of Runes 2 Mangara of Corondor 2 Aven Mindcensor 3 Flickerwisp 4 Stoneforge Mystic 3 Mirran Crusader 4 Phyrexian Revoker 1 Fiend Hunter 4 Thalia, Guardian of Thraben Lands (23) Spells (10) Decks like Ari Laxs Death and Taxes could use Mother of Runes to pressure opposing removal spells, protect ones self from opposing attacks, or just force through some damage
It isnt much to go on, but if you see someone with, say, a large amount of Unglued Plains, Mountains, Swamps, and Islands for trade but no Forests, alarm bells should ring in your brain