Forex signals are basically trading ideas. A signal system is used to determine whether to buy or sell a pair. The signal system could be executed manually, that is the trader observes the charts and analyzes where to place a buy/sell order or it could be automated through computer software.
In this case, the trader needs to set up the program to determine which signals it should be looking for. Besides an entry price, the signal system might also include stop loss and take profit levels.
Some companies and experienced traders offer to share their signals and charge a subscription fee in exchange, while some other services are free. These signals are often shared via email, a website, or even Telegram or Whatsapp.