Dimensions is just an idea that we created for understanding universe .we do not understand it completely,but we can understand it partially by assuming some criterias that is general .
Before 1900 ,everybody believed that time is absolute (a universal constant ).But einstein wronged that by relativity principle. The time depends on the velocity of a person .There are some assumptions ,but you can neglect that.His assumptions made people to think in different way.Before einstein it was thought that the space with three dimensions and time are different.But From his theory it was found that space and time are same in certain conditions like that in black hole.
The space is made of 10 dimensions as of now by scientists.with 9 space dimensions and 1 time dimension.
They are the multiverse theories and all those theories that you have seen in tv shows. But it is difficult to explain them.