Hello, just looking for some basic info on what it means trading with FOREX currencies in pairs.

What I mean is when I place a trade on EUR/USD, what am I actually looking for the currencies to do? Is it like a ratio type thing where if the EURO has a stronger day then the price will go up but if the US Dollar is stronger then the prince will go down? so it's like playing the EUR currency against the USD currency? when we select to trade EUR/USD

I understand that we buy and sell on the pair but don't really understand why they are in pairs. I can understand if it was one currency, lets say USD and if I think the USD will go up then buy and VISA versa for Sell but just don't understand when there in pairs like they are.

Please clarify for me.
Thank you