Exchanges are platform (markets) where cryptocurrencies can be traded using fiat currencies or Tokens. For most crypto holders, trading on exchanges is virtually inevitable. Crypto holders can purchase more crypto via exchanges or sell crypto for fiat currencies that are supported via the particular exchanges.
Exchanges are relatively safe but remember that it is always safer to store the majority of your cryptos in cold wallets and only transfer to exchanges the amount you wish to trade with or sell.
Below are a list of popular crypto exchanges.