To answer this question, find the "exchange rate" of these two currencies. Exchange rates are updated regularly and can be found on various web sites, such as xe.com, which offers an online conversion calculator. The reason the value of a particular money changes relates to the economy in the places that use the particular money, as well as other factors. When you exchange (or change) currency, you are essentially buying another type of currency with your own currency.
How many dollars are in 5,000 pesos?
Let's say you visited Mexico on vacation and wanted to purchase an item that cost 5,000 pesos, you would need $460.63 in US dollars to cover the expense. We can convert the 5,000 pesos into US dollars by knowing the current Mexican Peso to US Dollar exchange rate (which, at the time Cliff wrote this, is 0.0921527) and multiplying the number of pesos and the exchange rate:
5000 multiplied by 0.0921527 (current exchange rate) = 460.629 = $460.63 U.S. dollars
As you can see, we rounded up our money to the nearest cent, since the half-cent is rarely used in any currency. Also note that many countries use the term "peso" for the name of their currency, but all pesos are not equal. For example, the Mexican peso (MXN) is worth more than the Chilean peso (CLP).
Again, note that these exchange rates change on a regular basis, and that the exchange rates listed in these examples may vary (sometimes dramatically), within years and even months. The 5,000 Mexican peso item that you intend to purchase may cost more (or less) than it did today, depending on the exchange rate at the time you plan on making the purchase.