TL;DR: A bot is a tool for automating a trading strategy.
Traders buy and sell assets according to certain rules, according to certain trading strategies. Almost any trading strategy can be described as simple rules, a set of algorithms (for example, look at the price, if it is below X, open a buy order, if it is above Y, open a sell order).
Such processes can be quite routine or difficult for a human to perform.
Bots are programs that will tirelessly perform these processes day and night, monitor the price automatically, open and close orders automatically, and do it much faster and more efficiently than a human.
A good and working bot is a bot that can perform the desired set of actions, i.e. implement the trading strategy necessary for the trader.