A "transition word" or "transition phrase" are words/phrases that are used to show the relationship between clauses, sentences, or even paragraphs. They help us created cohesion by illustrating how different ideas relate to each other based on different elements, including time or sequence, comparison, cause and effect, contrast, emphasis or direction. A transition word can be an adverb or a conjunction (coordinating or subordinating).
When using transition words or phrases in German, we need to be careful with how the word order may change. For example, if a subordinating conjunction functions as a transition word, we need to remember the word order of a subordinate clause. If we put a transition word (or any element really) in first position or a sentence or clause, we create inverted word order.
Ich fuhr nachts nach hause. Auf einmal sprang ein Reh auf die Strasse.
I drove home at night. Suddenly a deer jumped onto the road.
Ich fühle mich heute nicht so gut, deshalb bleibe ich heute zu hause.
I don't feel so good today, therefore I'm staying home today.
In the two examples above, the transition words were placed in first position, creating inverted word order. But transition words can also be moved to different parts of the sentence.
Ich habe zukünftig keine Zeit mehr. I do not have time in the future.
Er hat mittlererweile keine Lust mehr. Meanwhile he doesn't feel like it anymore.
Here is a list of additional transition words or phrases and their common (but not exclusive) purpose: