how do reinforcement and boost work? and can you target hidden characters with effects?

Reinforcement is caused when a character is attacked and a support row character either adjacent or behind the attacked character exhausts for reinforcement. The cost for reinforcement is that other character's exhaust. When you reinforce, it causes for the attacked character to not take any breakthrough whatsoever, no matter what the out come of the battle. If they're stunned, you will still take stun endurance loss, but unless a card is played that stops reinforcement, you will not take any breakthrough endurance loss whatsoever.

Boost usually has a number next to it. When you want to boost a character, first you must recruit that character and pay it's recruit cost. So let's say you want to recruit a 2 drop character like Beetle, so you pay the two resource points. But, the card has a boost effect. To activate the boost effect, you must pay extra resource points equal to the cost of that boost. Now Beetle has a boost of 4, which means that when he's recruited you may pay 4 more extra resource points to activate his Boost effect. But you must have this many extra resourse points, and in all if you recruited Beetle with his boost you'd need to spend 6 resource points (2 for his cost plus 4 for the boost).

Boost is just the extra resource points you spend to activate a characters effect that comes after the Boost keyword. Also, you can only pay the resource points for boost as the character is being recruited, so you can't just recruit the character one turn and then next turn pay resource points to his boost although he's already on the field.

I hope I've been helpful.

From the comprehensive rules...

507.1c A player may play a reinforcement game-based effect that reads "Target defender you control has reinforcement this attack if it shares a team affiliation with and is adjacent to the exhausted character," with a cost of exhausting a support row character that shares a team affiliation with and is adjacent to the defender. (See rule 701.10a.)

"Boost" is a keyword that represents a continuous power that functions in any zone the card can be played from. The phrase "Boost <cost>: <text>" means, "As an additional cost to play this card, you may pay <cost>. If you pay, this card has <text> in addition to any other text it has." If you did not pay, the card does not have <text> in addition to its other text. All boost costs are expressed in resource points unless noted otherwise.

Thank you for explaining that so much better than I could have.

For the second question, you may target hidden characters with effects. The only exception would be if an effect specifically states it can only target visible characters.