Those are Hero Talents, basically they are buffs for you Hero, for example the Talent: Art of War adds up to 3 more mercenaries to you're hero, BattleFever increasing ATK ect. Each Talent however has different levels with superior or inferior effects, for example a 1/3 Art of War will have a weaker effect than a 3/3 Art of War, and that applies to all talents. Buy paying jewels you will get a random set of 8 talents of various levels to choose from, you can pay jewels to choose a talent or pay jewels to get a new set of talents to choose from if the talents were not satisfactory. They are Worth it, but do not invest in them unless you plan on using the Hero as a Primary hero! And I strongly recommend you have a STRONG team of heroes BEFORE investing in Hero Talents.
The 50k Souls Mercenary Talents are worth it, much better than, leveling them. Once you get your Mercenaries to lv 30 I strongly recommend you get the talents instead of continuing to level them up.