Trading means “exchanging one item for another”. In the financial markets, it’s buying shares, futures, options, swaps, bonds, etc… or like in our case, an amount of cryptocurrency.
The idea here is to buy cryptocurrencies at a specific price and sell it at a higher price to make profits (even if you can still make a profit if the price goes down).
To make it simple, the value of an asset changes due to supply and demand: if a lot of people want one bitcoin, the price will go up. If a lot of people want to sell one bitcoin, the price will go down.
Nowadays, thanks to exchanges like Coinbase or Kucoin, anyone can start trading using the tools provided by those platforms. They even offer API allowing you to create bots (with Cassandre, for example), which automatically do the trading for you based on the rules you choose.