sounds sooooo simple, doesn't it!
but think about it: if it were so simple, everyone would be in the Warren Buffet class.
very few are so there must be something not so simple there.
Then consider the idea that those who gained the most during the California gold rush were the merchants, not the gold miners. With stocks, it is the folks managing the trades and transactions (e.g. Wall Street) that most consistently make a profit because they sell a service and don't gamble on unknowns.
The pension funds are also worth considering. They buy the services of the smartest people they can find to "buy low, sell high" and generally do get an overall rate of return better than most index funds ... but nothing like returns even 20% or higher as suggested in some comments.
The Facebook IPO is a reminder of the heyday of the VC craze back ten or fifteen years ago. Talk about buying low with a promise of immediate large gains! The problem is that many of the companies going public were more like Solyndra than Google or Microsoft or, maybe, Facebook.
Short term? Good luck. Too often it is like the lottery with the odds against you.
Long term? You can spread out the risk for better odds. You will need a model of some sort. You will need some way to determine when "low" is not going to go "lower" and when "high" is high enough.
Your model can be random and depend upon economic growth as the basis. It can be based on existing successes in an index fund which, again, depends mostly upon overall economic growth. It can be based on fads (like Tesla) hoping they will grow out of the 'fad' phase. The benefits tend to go with the risks.
What you do (or should) know is that investing in stocks is investing in the potential for making wealth, making something out of nothing. That is growth. By investing, you hope to share in that growth. The capital investment is only one of the ingredients in that recipe. The other ingredients, market, people, and government, all have their influence on the wealth creation as well and it is those 'other ingredients' that make things 'not so simple' and the investment of capital an activity where massive gains are not assured. What can be assured is that, if an economy grows, your investments likely will as well if you spread them out a bit to spread the risks. So they question "are you still in?" is really a question about whether or not the world is going down the tubes or will the problems be fixed and lessons be learned and the miracle of the last 150 years continue?