what is quantum foam?is it possible to travel to the past using it?

Answer 1

"Quantum foam" refers to the constant creation and anihilation of particles issued from "nothing".
Spontaneously, COUPLES of identical particles are created, one particle and one anti-particle (hence conservation of total energy).
These particles are immediately attracted by each other and tend to rejoin, when they anihilate.
On the edge of a black hole, one of the two particles can fall below the event horizon and be captured by the black hole. When this happens, the other particle is free and seem to have been created out of nothing, violating the laws of energy conservation. This is why the black holes appear to "create" energy.
The total entropy (universe), however, remains the same: the black hole looses energy and "shrinks". That is why black holes gently evaporate...
Attempting to use this phenomenon for time travel is not practical in any way to do anything!

Answer 2

Quantum foam is the smallest, most basic and irreducible level of reality. Picture it as an unending jittering of virtual particles (and virtual space and virtual time) appearing and disappearing for no discernable reason out of the vacuum energy. Since this energy can be either positive or negative it does not violate conservation laws on average.

Because it underlies everything all the time, I don't see how anyone could make it do anything useful or exotic, especially time travel.