"They bought two new cars and a boat last month" If this month is April, then they bought two cars and a boat in March. "They've bought two new cars and a boat in the last month" If today is the 15th of April, then between the 15th of March (or around then) and today they bought two cars and a boat. If you're still confused, look at the different tenses ie. "They bought... last month" and "They've bought...in the last month".

"In" that period of time. We're not talking about a calendar month, but in fact the 30 days (approximately) leading up to now. Notice the present perfect structure?