Transitions are words or phrases that connect ideas. They are almost always found in the beginning of a sentence and help an essay flow so that the reader can better understand where the essay is going. It can also help tie up the point before it, so that the argument is strong and complete.
There are different types of transitions:
1) Transitions that describe or lay out a sequence/order. Examples: First, second, then, next, Later, Finally, etc
2) Transitions that add to an earlier point. Examples: Furthermore, In addition, Additionally, Moreover, etc
3) Transitions that contrast an earlier point (the point before it). Examples: On the other hand, Alternatively, In contrast, Despite
4) Transitions that give emphasis to point: Especially, Clearly, Particularly, Specifically, etc
There can also be words like Generally or Usually that may not fit an above category, but can be helpful when organizing your thoughts and making an argument.
I hope this helps!