Have you ever been on a date with someone who won't shut up about themselves? They barely take time to breathe between talking about their job, their cat, their car, their exes, blah blah blah. They chatter so much that you can't get a word in edge wise.
If you've been unfortunate enough to get caught in one of these experiences, you know they're excruciating. They're enough to make you want to run screaming from the restaurant, bar, etc.
With this in mind, take a look at a webpage, a blog or something else that you've written. Do you have sentences that take up 3-4 lines on the page? Do you have paragraphs that go on for so long that they look a bit like the monolith in 2001?
If you answered "yes," guess what--you're being that bad date for your readers.
Staring at a computer or tablet screen for extended lengths of time can wear your eyes out (we know--we do it for a living). If a visitor to your website has to go over a sentence or a paragraph 2 or more times, you risk losing that person's interest.
It won't matter if your grammar is perfect (which it should be, of course) or you have something they'll want (ditto), if people have to strain their eyes too much, they'll move on.
Think of your page or blog as a sample or a taste test. Give readers just enough information to pique their interest. As much as possible, pare your sentences and paragraphs down to make the info easier to digest. If your readers want the full meal (ie. if they want more details on your products and services), they'll contact you and tell you so.
Give your readers big chunks of text and they might choke. Give them something short and sweet and they'll want more.