Yes, if you "sell short" then you borrow the stock and thus owe the stock back to the broker, so you must replenish the shares via buying them back to close the trade. There are two outcomes that will happen:
1. The shares will drop, and you will make a profit when you buy them back (just the reverse of buying the shares and selling them higher).
2. The shares will RISE, and you will suffer the pain of a "short squeeze" and you will lose.
Many hedge funds made huge short bets on Tesla when the firm was having trouble. The stock was languishing in the mid 40's to 50's. Then, when things turned around, the stock started to go up, and up, and up. In other words, the stock created a huge short squeeze that forced the shorts to "cover" their position.
Tesla ended up rallying from $50 to nearly $200 in a few months, and flushed out every short seller. It was one of the classic short squeezes of 2013.
The major indices trend UP over time, so unless you know the theme for why the individual stock will drop, it's best to stick with the 1x short ETFs as S2007S mentioned, such as SH (S&P 500 1x inverse ETF), or use put options to minimize your risk, rather than shorting the equity outright.
If you're just daytrading, then it really doesn't matter since you're only looking at price action for a single day, thus you can sell a stock short all day when the market is down and cover the position before the close.