Source: What level of protection does the free version of Malwarebytes offer?alvarnell said:In general terms, the Premium version will detect a malware installer or malicious application as it arrives on your computer and will prevent it's installation or launch, so no damage done.
If you manage to install the malware process/application the free version will not prevent it from running or being launched, so if it's spyware then it will harvest the information it's after and attempt to upload it. You would have to run a manual or scheduled scan before that process or application runs or reports in order to prevent it from doing whatever it's designed to do.