Slow-twitch (type I) fibers are fatigue-resistant and produce force slowly, while fast-twitch (type II) fibers produce force quickly and tire quickly.
Everyone has both, in a ratio that is largely genetic and roughly 50/50 in most muscles. Postural muscles such as the soleus lean slow-twitch; the hamstrings and the triceps lean fast.
The training implication is smaller than the internet suggests. Both fiber types grow with resistance training, and both are recruited once a set gets close enough to failure, because motor units are recruited in order of size. That is why a set of 20 taken to 1 rep in reserve builds size much like a set of 8 does.
Where it matters is force and speed. Fast-twitch fibers are the ones that produce a maximal single or a fast first step, and they need heavy loads or fast intent to be trained hard. A client whose whole program is sets of 15 at RPE 7 is training the ones they can already use.
Fiber type cannot be measured without a biopsy, so ignore the questionnaires that claim to tell you. Program for the goal instead.