A siddha is one who has attained siddhi, i.e. "power, prowess, strength, ability", then a special kind of psychic and supernatural, miraculous, occult power.
Siddhas are great saints who lived and continue to live, during various times. They are beyond the mundane things. They seek only the attainment of salvation through persistent prayer to Lord Siva. They are alchemists, who also gained various powers due to their penance. They can turn any metal into gold; walk on water; fly through air; enter into a dead body and bring life to it; can bury themselves underground or submerge under water for years together. Like this the list can go on. But more importantly, they invented great medicines that ordinary humans cannot even fathom to understand other than realizing the benefits of it. They know the perfect combination to make life-saving medicines that could have been made from highly poisonous plants.
Some common questions about Siddhas:
Are Siddhas magicians?
How can Siddhas live so many years?
Do Siddhas still live in this world?
How many Siddhas are there in total?
Are there difference between Siddhas and Maharishis; if so, what are they?
Is it scientifically possible to turn any metal into gold?