Skill Points (SP) are used by characters to cast skills. Some characters want to use their skills all the time, generally consuming SP each turn. These include most dps units such as Jing Yuan, Himeko, and maybe some supports such as March or Yukong. Perhaps the most extreme examples of this are Qingque and Lunae, who can eat up all your SP in one turn.

Other characters do not necessarily consume SP every turn or can restore SP after consuming them. These include Seele, Blade, Bronya, etc. Such characters may be called 'SP neutral'.

And then there are some characters that typically generate SP each turn for the rest of the team, not having to use their skills all the time. These can include Silver Wolf, Topaz, Luocha, most healers, Tingyun, etc. Such characters may be called 'SP positive'.