When something sustains something else, it supports it and keeps it going.
If something sustains something else, it helps it to stay strong and not stop.
Examples:
The water sustains the plants.
She sustains her friend when he is sad.
The bridge sustains the weight of the cars.
The love of his family sustains him through hard times.
The superhero's power sustains the city from harm.
The sun sustains all life on Earth.