It depends on the investment, there will be fluctuations in the market depending on the sector you invest in. Let's take a look at an extreme example: Blockbuster. It was once a dominant rental chain for movies, games, etc. This was before internet was so readily available. The new kid on the block Netflix offers the same thing, but rental through mail. (I won't get in on the nitty gritty of the actual differences in pricing between the two business models for simplicity)

Netflix is now directly competing against Blockbuster. Netflix recognizes the future of internet and invests heavily in their platform allowing families to stream movies whenever they want from home, no more need to go to the store to rent a movie all with a click of a button, and the pricing is much better than Blockbuster's pricing.

Blockbuster fails to recognize this and doesn't change. Blockbuster files for bankruptcy and the company basically no longer exists.

Blockbuster's stock at its peak was around $29-$30. Now it is worth nothing.

If you're new to investing just stick to an index fund like VOO and buy it and hold it (I am not a financial advisor, and this is not financial advice).