In TI reference design, why the gnd1 and gnd2 of isolater connect with a cap?
It's not really to do with the isolator- but you'd typically use a DC-DC converter with such an isolator, and the capacitor is usually needed to conduct switching noise from the switchmode power supply back to the input. 

Edit: See the document "Cantrell, Mark. Application Note AN-0971, Recommendations for Control of Radiated Emissions with isoPower Devices. Analog Devices." referenced in the back of the document AKR linked to- they use the term 'stitching capacitance' because it joins ('stitches') together two isolated ground planes for RF. Note that the capacitor is a single point failure for the isolation barrier so it must be a safety-rated type such as Y rated if a breach of the isolation barrier could cause a safety issue.