Hollstein is usually referring to the relationship, or ship, between Carmilla Karnstein and Laura Hollis.

From the beginning of Season 1 we understand that Laura and Carmilla have a connection even they don't understand. Soon, Carmilla's flirting and looks make Laura consider what would happen if they dated. After episodes of flirting and some private and cute moments, Carmilla makes the first move and goes for the kiss. Of course Laura feels the same way so kisses Carmilla back and THAT'S the start of a very unique and wonderfull relationship.