You can Recruit a Mercenary from the Mercenary Docks by choosing this action. You can Recruit a level 1 Mercenary for free, but to Recruit a Mercenary of level 2-4, you must play a single Mercenary card that is exactly 1 level lower than the Mercenary you want to Recruit.
Example: You want to Recruit a level 3 Mercenary. To do so, you play a level 2 Mercenary. The level 2 card remains on the table, and the level 3 card goes into your hand.
Level 1 Mercenaries can be recruited for free without spending a card, but you must still use one of your actions to Recruit.
Example: John starts his turn and has a Hy’dran Fish’Bean (blue level 2) as his Leader. This allows him to take a level 2 Mercenary from the Docks during his Starting phase, and also allows him to take the same action twice during his Action phase. John decides to play the card he just took to Recruit a level 3 mercenary with his first action. He then uses that level 3 Mercenary to Recruit again with his second action to Recruit a level 4 Mercenary.