What does it mean to "assert a pin"?
Asserting a pin means setting it to its active state.

De-asserting a pin means setting it to its inactive state.

If a pin is active high (which it is, in your case), then asserting it means setting it to logic high (usually 3.3V/5V) and de-asserting it means setting it to a logic low (0V).

On the same page of the datasheet you've linked there's Table 2-04 which shows that Asserting pin 9 (Sleep_RQ) means setting it high: