It's called 'fac' 'mer' isomerism. Because around the metal, the 6 ligands form an octahedron which can go in 3 different different directions so form 2 different isomers.
First you need to understand that there are 3 directions and really imagine them in your head this way, when you look at the complex.
Simple line --- plane of the paper
wedges --------bonds towards the viewer
dashed --------bonds away from the viewer
In the mer isomer, the three similar ligands define a meridian of the octahedron. Meridian is like the imaginary circle that runs north to south (like on the earth's surface, from the North Pole to the South Pole). So in the 'mer' isomer, we start, for instance, in the plane of paper, then the 2nd similar ligand is towards you, then the 3rd similar is again in the plane of paper. Imagine going through that, you will find you just made the 'mer' of octahedron.
View attachment 62140
Like in the image, the isomer labelled mer has Cl ligands in 2 different directions (in plane of the paper and going away from you)
Similarly, in mer isomer, the ligand H2O goes in 2 different directions.
genuine thnx
May u b blessed