How can players use the appraisal feature to determine a Pokemon's strength and potential in Pokemon Go?

When you first started playing Pokemon Go, you might have thought that CP was all that mattered in a Pokemon. That is what you power up and change to make your Pokemon stronger, after all. If you've been playing for a little while, however, you might have noticed the appraisal feature within the game. You can appraise Pokemon to see their individual stats in three categories: Attack, Defense and HP.
These different stats are divided into three sections, giving you a better idea of how good that Pokemon is in each category. You can also see a star rating for the total number of points there are in each category. It's worth noting that even if you catch two of the same Pokemon, they will have different rating. Pokemon that have full stats in each category are called hundos and go in a special portion of your Dex, which is unlocked after you have caught a set number of hundos. A Pokemon that has spawned on the map will have the same stats as everyone that catches it until it despawns.

If you have a Pokemon that has no stats in any category - which can happen - this is called a nundo. Though there isn't actually a place it appears in your dex, it's a funny thing to have and lots of trainers tend to share these images online, claiming it's rarer than a hundo.
But, what do these different stats mean? Attack is how much damage your Pokemon does when attacking in PVP or defending a gym. Defense is reflective of how much damage your Pokemon will take when being attacked - having higher damage can allow your Pokemon to get hit more times than those with lower damage, without losing all their health. HP is health, Pokemon with a higher HP will be able to gain a higher health when powering them up.
If you do have a Shadow Pokemon, it is worth noting that purifying it will raise the appraisal as 2 in each, though if you have maxed a specific stat, that will not raise beyond the max. Shadow Pokemon are generally stronger than non-shadow Pokemon, so it really depends on what you want to do.

Knowing what these stats are is the first step in building teams for PVP. It's only worth investing your Stardust and Candies into Pokemon that have stats that you are looking for. Though hundos might seem like the ideal choice, it really depends on what you want to do. When it comes to The Battle League, you might actually want Pokemon that have zero attack, but a lot of defense and HP, as that could make them more of a tank, keeping them out for longer draining shields. Some Pokemon do better with specific stats, but it does depend on what is going on with the current leagues.
Hopefully, this guide has helped you get a start in understanding the Pokemon Go appraisal feature - as it's well worth keeping high star Pokemon and getting rid of those that don't have useful stats in the long run!