Talent Boost events allows you to temporarily unlock additional Hero Talents or Weapon Mods to boost your performance in Missions.  

How to unlock Temporary Talents?
Temporary Talents and Mods are only available during Talent Boost Events. Each Talent Boost Events will be focused around temporarily unlocking a specific set of Talents or Mods for your cards. Temporary Talents and Mods cannot be activated outside these events.

These Temporary Talents and Mods follow the usual order and require you to first max out your existing Hero Talents and Weapon Mods. Activating each Temporary Talent or Mod costs Training or Upgrade tokens, with the exact cost depending on the card type and rarity. Note that activating Temporary Talents or Mods cannot be undone, your Training and Upgrade tokens will be consumed at the time of activation and would not be returned once the Temporary Talent or Mods expire.

Both Heroes and Weapons can be temporarily boosted and can have multiple Temporary Talent Boosts active at a time as long as the current Temporary Talent Boost event allows it.

Once activated, all Temporary Talents or Mods added to your existing Heroes and Weapon will display a tooltip timer. The timer will have a grey background when there is more than 24h left. When there is less than 24h left the grey will turn yellow, and at less than 1 hour left the timer turns red/orange.

The icons of these Temporary Talents or Mods have a gradient bar in the list. In the second image, we see one Talent with half of its time remaining (icon is half yellow), and one Talent nearing its expiration (icon is almost entirely red-ish).