Most metals on the periodic table lose a fixed, predictable number of electrons when they form chemical bonds, settling into one single, characteristic charge state across essentially all their compounds. Transition metals behave differently: many of them can lose several different numbers of electrons depending on the specific reaction, forming several genuinely distinct, stable charge states rather than being locked into just one, a flexibility that traces back directly to the particular arrangement of electrons transition metal atoms have available to lose.
Transition metals keep several electrons available at similar energy levels
In most metals, one small, clearly defined set of outer electrons sits available for bonding, while the rest are held much more tightly and essentially never participate in ordinary chemical reactions, which is exactly why those metals settle into one consistent, predictable charge state. Transition metals have a somewhat different electron arrangement, with a larger set of electrons sitting at fairly similar energy levels, meaning several different numbers of electrons can be removed with comparably modest energy costs, rather than there being one single obviously preferred number to lose.
Different reaction conditions favour different achievable charge states
Because several different electron counts are all reasonably achievable for a given transition metal, exactly which specific charge state actually forms in a given reaction depends on the particular reaction conditions and the other chemical species involved, rather than being fixed in advance for that metal regardless of context. This is precisely why a single transition metal element can appear in multiple different, well-characterised compounds with genuinely different charge states, each one chemically legitimate and stable under its own specific conditions, a variability most other metals on the periodic table simply don't exhibit to nearly the same degree.
What we're still unsure about
The basic electronic structure explanation for transition metals' variable charge states, and the specific charge states well-documented for individual transition metal elements, are precisely characterised, extremely well established inorganic chemistry. What's more genuinely an ongoing area of applied chemical research is predicting exactly which specific charge state a given transition metal will actually favour in a novel, not-yet-studied chemical environment, since this depends on a genuinely complex interplay of factors, the specific other atoms and molecules present, temperature, and more, that chemists continue to refine predictive models for, rather than having one simple, fully general rule that reliably predicts the outcome for every possible new reaction in advance.
This sits inside Transition Metal Chemistry, one of seven topics in Inorganic Chemistry, one of six domains in Chemistry, one of seventeen subjects the app can quiz you on.