Terror in politics is when some people use violence or threats to make others afraid and get what they want.
Imagine you have a toy that your friend wants, but you don't want to share. One day, your friend threatens to hit you if you don't give the toy to him. That's terrorism at a small scale.
Now imagine this on a bigger level. Sometimes, people use terror to influence politics. They use violence or threats to make the government or the people do what they want. This can be anything from releasing prisoners to getting their own way in a vote.
It's like a big bully trying to make everyone do what he wants by hurting them or threatening them. It is not okay to use terror to get what you want because it can hurt people and make them afraid.