A soft peg is a term used for countries with a fixed exchange rate regime. There are soft and hard pegs. Soft pegs generally let their exchange rate fluctuate through a desired bracket. Hard pegs follow the anchor currency more stictly.
What they mean is fixing the exchange rate between two currencies. A hard peg means that the country tries to keep it's currency at the exact same exchange rate as another currency (usually it is a country trying to keep its currency pegged to the USD).
A soft peg means the country tries to keep the currency exchange rate basically about the same, but allows it to fluctuate up and down a little.