"Seized" is the past tense of "seize" and it means "to take hold of." In general it is often used to mean "to capture." You can seize a person and that might mean to arrest them. You can seize property, such as what the government might do to a house if the owner is no longer able to pay for it. I'll show you an example sentence:
"After the people raided the evil king's castle, the people seized all of his gold to use for the betterment of their village."
I think from the context of this sentence, it should be clear to see the meaning.
The word "seize" as a verb can also refer to a person. This refers to a medical condition called a "seizure." When a person seizes, their body begins to convulse uncontrollably and I think they sometimes lose consciousness when it happens. I've never had it happen to me, but it can happen to those with epilepsy, brain damage of some kind, or for other medical conditions. It has to do with an abnormality in the brain. I'm not doctor though, so I can't explain it with complete clarity.
These are all the definitions that I can think of for the word "seize." I hope this helps! Let me know if you have more questions.