A drawdown is a decrease in the balance and equity on the trading account; to put it simpler, a drawdown is a loss. Drawdowns can be of two types: floating and fixed.
A Lock is several positions open for one instrument in different directions on one trading account. We shall discuss it on the example of Forex, as long as on stock markets, in the USA specifically, locking one instrument is illegal; one can only hedge risks by other instruments (buy one, sell another). Locking is also possible with CFDs.