Mana typically has a color, one of five: White (W), Blue (U), Black (B), Red (R), or Green (G)
Selvala's Stampede basically tells players to choose whether you get random creatures from the top of your library or permanents from your hand, and theyre also in charge of deciding how many of each you get
"Don't worry guys, I'm just going to counter this with Arcane Denial so we all draw cards!" This fuels the overall card flow while stopping only a single opponent's key spell
Technically, you can adapt as often as youd like, it just doesnt do anything if theres a +1/+1 counter on the card
They have similar restrictions to Pillar of Origins that make them less effective in gameplay than they might seem, but they're still great fixing for multicolor decks