I hear this pretty often when I say/write something, be it in text messages or emails or on any social media/forums. What does this mean actually? I am thankfully able to convey my thoughts but I do not want to hear this anymore. Are there any thumb-rules to follow when I say/write something to make it "sound natural"?
An important part of writing well is understanding your audience and using the words that will have the desired effect on them.
If you're writing text messages to a sixteen year old, 'natural' language is going to be a lot more casual and text-speak-ish than if you were writing them to someone unused to the medium and more practised at long-form writing.
Similarly, the level of diction on a writers' forum is probably higher than the level on, say, a forum dedicated to a pop group or celebrity popular with uneducated people.
"Natural" essentially means saying things in the way people expect to hear them. It's not necessarily a positive, but if you want to draw attention to your content rather than your manner of speaking, try to read how others post/write in that forum and imitate it.
Natural refers to language that native speakers say or write. It is possible to write sentences which are not natural, by using words that wouldn't normally be used, or using word order that makes sense but is unusual, or by speaking/writing in a way that is unsuited to the situation.
An example would be if a person always spoke as if they were dictating a legal contract, using only a small number of precise terms. Or if every phrase they said was a Billboard top 100 song lyric. Or if unusual word order like Yoda they use.
To make your writing seem more natural, you need to read more and practice writing sentences that match the patterns you see in real life. You need to develop a sense of which words to use and where, and which kinds of sentences to use and which to avoid. You need to have appropriate grammar and choose the appropriate dialect and register.
For what it's worth, what you've written in your question sounds perfectly natural to me.
"Natural" is based on what most people would say. A sentence could be perfect grammatically, and not sound "natural." This could occur in a situation where most people would use abbreviations or slang to say what you are saying.
Example: A "hypothecation" is a mortgage. A Spanish speaker might say, "I have a hypothecation (hipoteca) on my house. But most English speakers would use the French term, "mortgage," and say "I have a mortgage on my house."