A relative large number of intracranial diseases can show restricted diffusion and may therefore appear bright on diffusion-weighted images. An incomplete listing by category of disease is shown below, with the most common examples highlighted in red:

Neoplasms
The vast majority of neoplasms do not restrict diffusion or change it only mildly. A few interesting tumors, including lymphomas and some highly malignant gliomas, may show significant restriction of diffusion and thus appear remarkably bright on DW images.

http://mriquestions.com/dwi-bright-causes.html

Diffusion-weighted imaging (DWI) is a form of MR imaging based upon measuring the random Brownian motion of water molecules within a voxel of tissue. In general simplified terms, highly cellular tissues or those with cellular swelling exhibit lower diffusion coefficients. Diffusion is particularly useful in tumor characterization and cerebral ischemia.

Terminology
A great deal of confusion exists in the way the clinicians and radiologists refer to diffusion restriction, with both groups often appearing to not actually understand what they are referring to.

The first problem is that the term "diffusion-weighted imaging" is used to denote a number of different things:

isotropic diffusion map (what most radiologists and clinicians will refer to as DWI)
the pulse sequence that results in the generation of the various images (e.g. isotropic map, b=0, ADC)
a more general term to encompass all diffusion techniques including diffusion tensor imaging
Additionally, confusion also exists in how to refer to abnormal restricted diffusion. This largely stems from the initial popularisation of DWI in stroke, which presented infarcted tissue as high signal on isotropic maps and described it merely as "restricted diffusion", implying that the rest of the brain did not demonstrate restricted diffusion, which is clearly not true. Unfortunately, this shorthand is appealing and is more widespread than using the more accurate but clumsier "diffusion demonstrates greater restriction than one would expect for this tissue."

https://radiopaedia.org/articles/diffusion-weighted-imaging-...

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Note added at 1 hr (2019-09-08 19:05:32 GMT)
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Diffusion-weighted magnetic resonance imaging (DWI or DW-MRI) is the use of specific MRI sequences as well as software that generates images from the resulting data that uses the diffusion of water molecules to generate contrast in MR images. It allows the mapping of the diffusion process of molecules, mainly water, in biological tissues, in vivo and non-invasively.

DWI showing restricted diffusion in the mesial dorsal thalami consistent with Wernicke encephalopathy

https://en.wikipedia.org/wiki/Diffusion_MRI