"Dimension" is used in mathematics, chemistry, physics and cosmology. Especially within physics and cosmology, the concept quickly becomes difficult to understand.
In short, we live in 3 dimensions: length, width and height. The time is often referred to as the 4th dimension. See the illustration below.
However, modern cosmology suggests up to 11 dimensions.
Higher dimensions will allow alternate realities and parallel worlds. Such a condition is a so-called "domain". A domain cannot be left and one cannot cross into other domains. In the 10th dimension everything is conceivable. However, before one wants to live the life as a millionaire in a higher dimension, one must know that the laws of nature are not necessarily the same. So if you found a way into another dimension, you would be dissolved in a cloud of atoms.
The term "dimension" is also used for unexplained events. ("It's a foreign dimension"). It is theorized that ghosts and other energies live in a different dimension. However, they can still be part of our "domain". In that case, the ghost or the energy has to follow the laws of nature that we live under. That is, we will be able to easily measure and record them. If an energy visits us from a higher dimension, and outside our domain, they will have the same sad fate: Namely, dissolve while trying. However, if they have found a way to "survive" the journey, they will not need to follow our laws of nature. They may disappear through walls etc.
Science has succeeded in creating new dimensions, at the research center CERN, which is located in Switzerland. New dimensions, formed for infinitely short periods of time. These dimensions collapse rapidly, and so far there is no indication that these are any danger. Whether or not someone or something can cross dimensions is uncertain. As technology is today, man is unable to do so.
What happens, if you extend dimensions. But remember, these dimensions are only seen on a 2-dimensional screen.