https://www.science.org/content/blog-post/pfizer-s-good-news-world-s-good-news
A little bit of molecular biology background:
Most of you know that a gene codes for a protein, sometimes (but not always) with a transcribe-to-mRNA intermediate step. In high school, we are taught a mental model of that sequence being "A single gene is transcribed into mRNA (or, in the case of a virus, starts as mRNA), then mRNA that encodes that gene is translated by the ribosome into the corresponding protein"
In real life, it's a little more complicated. In many situation (especially in viruses), the individual proteins are encoded in DNA/mRNA in a way that clumps multiple genes together. These clumps are called "Open Reading Frames" (or ORFs), and a single ORF might encode a single gene, two genes, or a small handful of genes. (And trust me when I tell you I'm doing a little hand-waiving here to simplify this explanation).
After an ORF is translated into the folded up string of amino-acids that comprise the resulting proteins (which are all still bound to each other in a big clump), an enzyme (called a protease, "pro-tee-aze") comes along and cuts them apart from each other, in much the same way that we might use a pair of scissors to cut a cluster of grapes apart into individual grapes. These proteins, now freed from the clump, then go on to perform whatever their normal function is. In the case of a virus, their function is typically involved in viral replication.
A protease inhibitor is a small molecule that binds to a protease, preventing it from performing its function of cutting the proteins apart from the translated blob.
Proteins not cut apart from each other = proteins unable to perform their function = virus no longer able to replicate.