Transition words are words that help writers get from one place to another. They allow for movement, whether that is sequencing, or chronology, or directions. They make our words flow so that they don't just sound like a jumble of sounds or like one sentence is running into the next.
Transition Definition
"Transitions are words and phrases that provide a connection between ideas, sentences and paragraphs. Transitions help to make a piece of writing flow better. They can turn disconnected pieces of ideas into a unified whole and prevent a reader from getting lost in the reading." http://literarydevices.net/transition/