Can you please advise what is the name of this substance called? It is at the corners of the wall and between the gap of my bath tub and tiles ? Attached photo for your reference.
That is silicone, which is also known as caulking. It's used to seal water out of gaps and finish corners. Since a bathroom is a wet area, it is likely something similar to Selleys 300g Wet Area Waterproof Silicone Sealant - White. You might like to have a read of this discussion from @MinB about their successfully replaced silicone sealant. You should also find this step-by-step tutorial useful: How To Silicone A Gap.
Please let me know if you need further assistance or have questions.