English has nouns (cat, fish, diaper) and verbs (scratch, swim, poop). A proper English sentence contains at least one noun and one verb ("The cat ate.''). Hopefully, the cat did not eat poop. Again.
In algebra, expressions act much like nouns with the verbs being relations, which are symbols that say how one quantity relates to another, such as =, <, ≥, and so on. An algebraic sentence is a statement of relation between two quantities, such as 2x = 4, which could be read as "two times x equals 4." Put into a phrase that sounds like something you might actually hear, we could say, for example: "I have 4 Christmas presents, which is 2 times as many as Xavier got." We wouldn't brag about that though. Sounds like you both got shafted.
Now that we've got the hang of expressions, it's time to put them together into sentences. So to speak.