Chain falling off front sprocket
Worn chain or chainring can cause this, as can a very dirty or un-lubricated chain, especially is there is a stiff link pin. A bent chainring can also buck the chain off. 

Another possibility is the derailleur is not providing enough chain tension. This can happen if one of the tensioning springs has broken or has become weak. Check the 'B' spring (at the attachment bolt) or the 'A' spring (where the cage pivots).

Unfortunately the tendency to drop the chain may just be a 'feature' of an inexpensive bike. You might look into fitting a chain tensioner to keep the chain on. Modern 'one-by' drive-trains with a single chainring use 'double-wide' chainring teeth profiles which are much better at retaining the chain. However, this is not an option on inexpensive bikes where the chainring is integral to the cranks.