"Heavy multitasking" refers to the act of simultaneously performing multiple tasks or activities that require one's attention and focus. Full definition
For those who are still concerned there is close to 1.2 GB of free RAM at launch and even with heavy multitasking, it rarely dips below 800 MB. (fonearena.com)
It is not meant for heavy multitasking and therefore, we didn't expect much out of it. (phoneradar.com)
The other smartphones we mentioned above have double the RAM which makes them a better option if you perform heavy multitasking. (pcquest.com)