Tailwind
This is Janna's passive, it allows allies to move quicker while moving towards her in games by 8%. There is a second part to this passive, for example depending on the runes you take or if you buy boots or movement speed items with this champion specifically, it will grant you more damage to your auto attacks and abilities.
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✽✽✽✽✽✽✽ This is a full game as Janna support with Ezreal ADC explaining the matchup, positioning,

and shot-calling:
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Resolve
Guardian is a good rune to take at the moment for protecting your allies in lane since you periodically get a shield for standing next to them.

Font of Life is a simple rune that allows your allies to heal for a percentage of the damage that they do to enemies who you have used your crowd control abilities on (your knock-ups such as your Q Howling Gale, slows like your W Zephyr or Twin Shadows, etc) will all allow your allies to heal when they attack an enemy champion. The healing is based on your max HP so it's more useful on tanky supports who provide CC (Crowd Control, like stuns, polymorphs, or roots). Please note that the Mobafire site isn't updated to show Janna's proper W "Zephyr" ability in the text.

Bone Plating
When you pick this rune, this gives you an extra shield upon taking damage. If an enemy champion deals damage to you, you take less damage from their next three spells or attacks for a duration of 1.5 seconds. A good thing to keep in mind when picking runes is the fact that some do in fact have a cooldown and duration.

Revitalize
This rune makes your heals and shields stronger and even more so when your teammates are below 40% health.

Sorcery

Manaflow Band and Celerity
I believe these are must-haves for Janna since they synergize very well with her kit. Manaflow Band provides permanent bonus mana throughout the game. The Sorcery tree feels like it was made for Janna; simply put you can get free movement speed and damage just for picking one rune option, Celerity, since once again it synergizes well with her passive.

Janna's Q - Howling Gale is a knock-up that can be charged to travel further and knock enemies up for a longer duration. The majority of the time casting it early near enemies provides a quick knock up that is sufficient for completing small trades.

Janna's W - Zephyr This is a very effective slow, however when you use this you lose the passive benefits of this ability while it is on cooldown.

Janna's R, Monsoon, creates a large area that heals herself and all surrounding allies until the cast is over, Janna gets hard CC'd (Crowd controlled by a stronger version of a normal effect such as a slow. Hard Crowd control are stuns, knock-ups, polymorphs, and roots) or dies.