Traditional Investment Advice:
Buy low sell high means you should buy into investments when the market is low, and sell those investments for gains when the market recovers.
Think of it like buying stocks on sale. You're getting reduced prices for buying a portion of great companies!
The biggest issue with buying low and selling high is that you are essentially timing the market, which most experts agree is a bad thing to do
You never know when the market will bottom out, or where the highest peaks will be
Another disadvantage is that it doesn't take your investment timeframe into account. You may need the money before the market recovers.
If you're investing in individual companies, be careful not to buy low when a company is on the verge of going under
Why is Buy Low Sell High Seen as good advice then?
Simple, short pieces of advice are easy to digest. Generally, buy low sell high is decent advice. But don't forget that most easy advice comes with nuance!
For more information, including what you should do instead!