Alignment
As described by the Pathfinder 2e Core Rulebook.
Good & Evil
Characters who align as good consider the happiness of others above their own. They're the kind to carry out selfless acts for others, regardless of association. Extending to protecting others from harm, even at the expense of the character's own safety. Evil-aligned characters on the other hand are selfish. Willing to victimize, purposefully or by coincidence, others in order to attain their goals.
Law & Chaos
Characters who value consistency, stability, and predictability, lawful characters have a set system in life. These can be the meticulous planning of daily activities, following official or unofficial laws, or adherence to a code of honor. Lawful characters believe that chaotic characters are irresponsible and flighty. Meanwhile characters who prefer flexibility, creativity, and spontaneity then they are chaotic. Chaotic characters often believe that lawful characters are too inflexible to judge each situation based on its own merits or take advantage of opportunities.
Alignment Changes
During the course of play a character's alignment can change, reflecting how one's beliefs may become altered through experience and by actions. For the most part, your alignment changing won't have much effect on how the game is played. However, if playing a class such as a Cleric or Paladin (Champion PF2e) or any other divine caster, should your character's alignment change to one now supported or allowed by your deity, they will lose their class abilities until they can atone or gain the favor of a new deity within their new alignment (atonement).
About Chaotic Neutral
To quote a friend and fellow old school player/GM of mine,
As stated in both the 1st and 2nd edition (D&D) Player's Handbooks, the Chaotic Neutral (CN) person is merely an insane person and has no compass of direction. This author disagrees. A CN person does not believe that having a philosophy built solely upon either good deeds or evil acts is logical, or better said, practical. Especially in the sense that one would allow such “directives” be the basis for actions throughout his entire life. As shown throughout the history of the worlds, laws simply better the lawmaker and having a set of universal laws is also impractical, seeing that there are many different peoples with many different aspects; how would they all agree on anything? - Dungeon Master Roccanti
So long as the end result does not infringe upon the Chaotic Neutral character's personal freedom, they're the person in the party to be the least bothered. This individual will take their blade to the field of battle only if something is perceived to negatively impact their lives. They will accompany a Lawful Good character. They will just as readily accompany a group of Chaotic Evil individuals. However, if the player of a chaotic neutral character simply uses this alignment as an excuse for acting like a murderhobo, needlessly harming NPCs, fellow party members, or otherwise acting in such a way that would be perceived by any rational person as 'evil' (you know the type, "it's what my character would do," the edge lord, etc), then it is advised you remove that player from your table immediately.