A life lease is just that it is a lease based on the life on a particular individual. For example, if I have a life lease on the property I am using that means I can continue to lease it until I die, then the property reverts back to the orginal owner.
A life lease can also be based against another persons life. For example, I am leasing a property, the length of my lease in this example is based on the life of Joe, as long as Joe is alive I may use he property, once Joe becomes deceased my lease expires with the person whose life the leae is based on. It does not have to be the tenant or the owner.
This is often used on income producing type properties where the tenant makes his living off the land, like a farmer, but not limited to this type situation.