Transitions are words, phrases, and even sentences that make a clear connection between two ideas, sentences, or paragraphs. Their main purpose is to make two or more ideas flow smoothly and better.
There are many types of transitions, such as those that express contrast (like on the contrary, however, notwithstanding), that denote time (after, at last, before), that add information to a previous point (furthermore, besides, moreover), that show similarity (likewise, similarly, in like fashion), that illustrate a point or give examples (for instance, for example, to illustrate), among others.