What does it mean to devalue currency? And why is it done?

As was mentioned bye Tallpowerhousblonde it is great for exports. That is part of why the Euro worked out so well for Germany although there were other factors too.

Unfortunately it can also be destructive. It can eventually turn into hyper inflation. The US is in danger of this partially because of their monetary policy of trying to fix economic issues with monetary policy. Part of how they got away with it is by funneling surplus dollars through other nations for international trade. More and more nations are deciding to make transactions in local currency instead of USD. Eventually that will catch up with the US as all those dollars come back stateside. With the current situation economically that will be ugly when it eventually happens.

"Devaluing" a currency is specific to the value of one currency against another , or the exchange rate.
So how many pence sterling or Euros or Yuan you get for your $US dollar for instance.
There are two ways this is set. First by the market, by supply and demand. Thats how most Western countires do it. Second by govt policy in a centralised economy, where the Govt controls the money supply and therefore the selling price of its own currency.
Thw Why is very complex there...But its usually tied to govt trade policy somehow,

Money is worth less.Great for exports and makes imports expensive.

Back in the Roman times for example.. they woyluld cut a slip of all the coins.. melt them down and make more coins.. thus lowering the purchase power

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