There are two primary things to remember about Civics:
- Your civilization can only have one Civic enacted at a time.
- Each Civic can only be claimed by one civilization at a time.
Now that I've told you that, guess what? There are two exceptions to the above rules. Meritocracy and Closed Borders are not "exclusive" - they can be enacted by all the civilizations in the game if necessary.
Most of the Civics give you a +25% bonus in one category (like Production) and a -25% penalty in another (like Gold). Some will also affect your citizens' Happiness, which makes all your citizens slightly more (or less) productive.
Meritocracy and Closed Borders are intriguing - they appear to have a benefit without a drawback. Look at Meritocracy, for example. It automatically removes inactive players from your civilization, which helps your civ pass votes in a timely fashion and provides space for active players to join. And hey, it doesn't give us -25% to anything!
The thing to remember is the opportunity cost of having one Civic - you're not just paying the penalty associated with that civic, you're also paying the price of not having another civic in place. You're forfeiting the +25% in some category that you could be enjoying. This is particularly true of things like Meritocracy (once your inactive folks are cut out, you're not gaining much and you're forfeiting a +25% somewhere else), but it's true of every Civic.
Think about the civic currently enacted in your civilization - is it the best one for your current situation? Or could your civilization be benefiting more from a different bonus or a different penalty?