The Potion of Life was a chemical mixture with the ingredients of its creation located within the Secret Books of Saxon and was a vital element in bringing about the resurrection of a deceased being.
The secret disciples of the Master during his time as Harold Saxon used the contents of the Book of Saxon to prepare his rebirth after he was killed by his wife Lucy Saxon. After a few years, they eventually began their plans which involved taking over the prison facility where Lucy Saxon was held and taking a biometric impression from her lips which was one of the elements needed for the resurrection. In addition to that, the Master's ring and the Potion of Life were gathered together with the final requirement being the life force of the Master's disciples which succeeded in bringing about his return.
However, Lucy Saxon added an extra ingredient within the mixture as she knew of the "ritual" with this chemical creating an explosion that created an imbalance within the reborn Master as his lifeforce was slowly dying. (TV: The End of Time)