There are 6 different attributes in game that every pokemon has, each contribute to how much power your pokemon has, there are various ways to increase these attributes that other pages on this wiki will cover.
Most Pokemons are either P-atk based or SP-atk, you can usually find out which one your pokemon is by hovering over their skills or passive and it will say deal X type of damage or increase X type of damage in passive.
There are a few pokemons that use both however, such as genesect, Lucario...etc.
hp (Health/hit points) Determines how much damage your Pokemon can take, this is the most generic form of defense. It is useful against both physical attack as well as special attacks.
1.5 Power per HP gained
Speed is widely considered to be the most important stat in this game, it is also the most expensive stat to improve.
physical attack deals damage based on enemies p-def, the higher the p-def enemy has the less your attacks will deal.
Special attack deals damage based on enemies sp-def, similarly the higher the sp-def the enemy has the less your attacks will deal
P-def is most likely the least focused on stat because there aren't many meta p-atk based pokemons, in addition to this, p-def is generally a lot easier to gain as stat as opposed to other stats.
SP-def is arguably the third stat you want to focus on after your speed and type of attack dependant on your team as most meta teams are sp-atk based.
the table below will tell you how much power you get per point of the above attributes