Soft reset and Hard reset are two different methods how to restart a mobile phone, tablet or other device. Both can be used to solve some types of software issues.
The difference between them is:
Soft reset - to do a soft reset, you have turn off the phone and then turn it on after a couple of seconds. On some phones there is also an option to press the Power button and then tap on Restart. In case of soft reset, no saved data should be deleted.
Hard reset - is also known as a Factory reset. It is complicated than soft reset, because it restores device to factory settings. It doesn't delete absolutely everything, but you should expect it will delete your data (photos, videos, documents, downloads etc.) and installed applications. So before hard reset, it is recommended to backup the data you don't want to lose. Depending on device, when trying to perform a hard reset, you should see the list of applications that will be deleted.