Chiral isomers are very subtly different from one another. They differ only slightly in their 3D appearance. So why does this matter?
All living things are carbon-based. This means that almost everything in their body comes down to interactions of organic (carbon-based) molecules. Carbon likes to form 4 bonds. If it forms 4 bonds with 4 different groups, it immediately has made itself chiral. The resulting molecule now is distinctly left- or right-handed. Many many important molecules in your body have a certain chirality. And it all matters because your body cares about chirality!
Of the ~20 amino acids in the human body, 18 of them are left-handed. This means that all the proteins in your body are connected in a way where chirality matters. And most proteins are shaped the way they are as a result of the chirality of their amino acids.
Many drugs work by fitting into pockets of proteins or enzymes and stopping them from working. The better the drug fits, the more effective it is. Proteins are 3D molecules, and chirality makes a big difference to them. A left-handed drug may have little to no effectiveness, but if you make it a right-handed drug it will work perfectly. The photo above is a rough schematic of this. Try to see if you can rotate the molecule on the right and get it to fit in the pockets as well as the molecule on the left.