A mini dental implant is exactly what it sounds like; they are dental implants that are a little bit smaller than regular dental implants. They are also called small diameter implants, or SDIs. They work on the same principle as regular dental implants but are much smaller, and as such tax the jawbones a lot less. They are tailored towards people who cannot have regular dental implants due to some form of bone density issue, but who do not necessarily warrant a bone graft.
The replacement of front teeth is usually also done with small diameter dental implants, as those usually do not tax the periodontium in the front of the mouth unnecessarily, where it is much thinner than in the back to begin with. Front teeth also have much smaller roots as well, so it would make sense that the replacement of front roots would also be much smaller.
Mini dental implants are also used in orthodontics, as well as in denture stabilization.