The sentence length check assesses what length your sentences are. It calculates how many words you use in one sentence. It then calculates the percentage of sentences that are longer than 20 words. If more than 25% of your sentences is longer than 20 words you should shorten some of your long sentences.*
Readability is important for SEO. Because Google likes to show (and people like to find and share) high-quality content, which is informative, nice to read and easy to comprehend. Typically, a text that consists of many long sentences is hard to read, because long sentences are more difficult to process. A lot of people lose track of the meaning of a sentence when it’s very long. If a long sentence is followed by other long sentences the reader might fail to understand the meaning of a text completely. That’s not the experience you’d like to give a visitor of your site and not an experience Google likes to give their users either!
If the readability analysis gives you a red bullet for sentence length, you’re using long sentences too often. Click on the eye icon next to the check to see which of your sentences consist of more than 20 words. The SEO Onpage app will highlight these sentences for you. Go through these sentences, reread them and see which ones you can cut into 2 or maybe 3 sentences. It’s no problem if some sentences consist of more than 20 words, but try to keep the occurrence under 20%. Also, try to avoid using a long sentence after another long sentence. Try to mix longer and shorter sentences, this improves the readability of your text.
*These are the boundaries for English. If you write in another language the boundaries for maximum sentence length as well as the prevalence of these sentences may differ: find the exact boundaries per language here.