Thrust is a type of force. thrust is also called pressure.
....pressure is directly proportional to force.
...pressure is inversely proportional to area on which it is being applied.
so that's why the formula becomes....
P = f/a
P... pressure.
f... force.
a.... area.
let me give you example.
now we have seen many people sitting on bed of nails. the question is that why did nails don't get into his body and hurt him.???
now in nails bed there are about more than hundred nails. now here as the number of nails is increased the area is also increased. now according to formula pressure is inversely proportional to area ( as the area increases force decreases.)that's why the nails doesnt hurt the person. now think if we will sit on a single nail then it'll obviously hurt us. now by this it is proved that pressure depends upon area on which it acts.