Answer:
Period refers to a amount of time needed to complete a cycle, while, on the other hand, frequency is the number of cycles per unit of time. That means that the shorter time between the cycles is, the more cycles there will be. Imagine clapping your hands every two seconds. In ten seconds you would've clapped five times. But if you decrease the period, and clap every second, then in ten secons you would have clapped ten times. So decreasing the period increases the rate.
Another way to look at this is through an equation: T (period) = 1/f (frequency). That means that T•f= 1, which means that if any of them increases, the other must decrease (and vice versa) so that the product of multiplication remains 1.
