そういえば is a hypothetical structure in Japanese consisting of the adverb そう ‘like that’, and the ば conjunctive form of the う-Verb 言（い）う ‘to say’. Common translations include ‘speaking of which’, ‘by the way’, and ‘come to think of it’. These translations are actually very close to the literal translation of ‘if saying that’ in that they all refer back to the previous statement as being a trigger for bringing up a topic that the speaker wants to discuss.
そういえば will primarily appear at the beginning of sentences, before the speaker will then bring up the topic that they were reminded of.
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そういえば、この前（まえ）借（か）りた本（ほん）読（よ）み終（お）わったよ！めちゃくちゃ面白（おもしろ）かった！
By the way, I finished reading the book I borrowed from you the other day! It was super interesting!
そういえば、大家（おおや）さんに家賃（やちん）振（ふ）り込（こ）んでくれた？
Speaking of which, did you transfer the rent to the landlord for me?
そういえば、さっき駅（えき）でお母（かあ）さんの友達（ともだち）に会（あ）ったよ。よろしくだって。
Speaking of which, I met your friend at the station. She told me to tell you she said hi.
Caution - Due to
そういえば refering to something that has already been brought up over the course of a conversation, it will not be used to move on to completely unrelated topics in the same way that phrases like ‘by the way’ are used in English.
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中田（なかた）さん：今度（こんど）バッティングセンターに行（い）こ！
高橋（たかはし）さん：そういえば、藤田（ふじた）さんと伊藤（いとう）くんが付（つ）き合（あ）い始（はじ）めたって聞（き）いた？
Tanaka-san: Let’s go to the batting cage next time!
Takahashi-san: Speaking of which, did you hear that Fujita-san and Ito-kun has started dating? (Unnatural Japanese)
Fun-fact - そういえば is very similar in the way it is used to といえば, with そう simply replacing the direct quotation of と. Many textbooks will list both as a single grammar point, despite そういえば itself basically being a set phrase.