Transition words are words that help connect or link ideas.
Examples
Sentence 1:
My grades in school are very bad. Therefore, my mom does not allow me to play video games any more.
In this sentence therefore is placed in the middle of the sentence, this is a cause and effect transition word and it helps explain the effect of having bad grades.
Sentence 2:
Although my sister was very tired, she helped me with all my cleaning.
In this sentence although is the transition word. It shows contrast or difference, so cleaning the house is a contrast with the fact that the person was tired.