Most currencies "float." That means their value goes up and down based on how many people want to buy or sell them — the same way any price moves when demand changes. A few things drive that demand:
Here's a simple example. If investors around the world want to buy US stocks, they need US dollars first. That extra demand for dollars can push the dollar's value up, even if nothing about the dollar itself changed.
Not every currency floats. The UAE dirham is one that doesn't.
Even though the dirham doesn't move against the US dollar, it still moves against currencies that do float, like the British pound or the euro — because the dirham is only fixed to the dollar, not to everything else.
Say the British pound strengthens against the US dollar. Since the dirham is pegged to the dollar, the dirham weakens against the pound right along with it — even though nothing about the dirham itself changed. That means a trip to London, or anything priced in pounds, gets more expensive in dirham terms.
This also matters if you ever invest outside the US or UAE. If you buy stock in a country with a floating currency, your return depends on two things: how the investment does, and how that currency moves against the dirham. That second part is called currency risk.
The dirham has been fixed at AED 3.6725 per US dollar since the late 1990s, kept in place by the Central Bank of the UAE. A lot of the UAE's trade happens with countries whose currencies do float — not just the US — so this peg is what keeps prices at home predictable, even while other currencies move around it.