Basically, you can think of them as robots that make trades on your behalf based on trading rules you provide to them.
Here are a couple of examples:
https://cryptotrader.org/
https://www.butter-bot.com/
As human being, we all have a life away from coin trading, so we can't stick to the computer all the time to see if the market conditions are suitable for you to make a trade. So you provide this set of rules to a bot and give them access to your exchange accounts and they'll make trades on your behalf - so you don't miss the pump & dumps.
Based on how much you trade and how well you define your rules, the bot can be profitable or not.
But the examples provided above are free, feel free to try them out.
Always wanted to know more about bot trading.. If we want to run 24/7.. Do we need to have a VPS or..?
If you use services like the ones mentioned above, you don't have to host anything. Everything runs on their servers.