Sahil Sura
Ansys Employee

Hello Aleš Kadlec,
In the FEA algorithm. a body is divided into parts which are referred to as elements and the displacements and forces are usually calculated at the nodes while the stresses and strains are at the integration points.
When the elements get smaller or if they're placed such that they are discontinuous from the others like a reentrant corner or so, there occurs a phenomenon of stress concentration or artificially high stresses.
Thus if observed, the stress results change rapidly on refining the mesh. So usually one of the aims for a CAE solution is to be 'Mesh Independent'. You can get more information on the same from the following course-
Understanding and Dealing with Artificially High Stresses — Lesson 3 - ANSYS Innovation Courses
Specifying an Appropriate Element Size for Stress Analysis — Lesson 1 - ANSYS Innovation Courses

Hope this helps!