Grimm's Hydride Displacement Law is a big name but actually very simple to understand! You know how when you share toys with your friends, things can get complicated if everyone wants the same toy at the same time? The same thing happens with molecules, but instead of toys, we're talking about hydrogen atoms!
Different molecules (like water, methane, and hydrogen sulfide) all have hydrogen atoms that they can share or swap with other molecules. Grimm's Hydride Displacement Law tells us which types of molecules are going to be more likely to share their hydrogen atoms with other molecules.
The law says that if two molecules have hydrogen atoms that are bonded to elements that are similar, those hydrogen atoms will be more likely to switch places. On the other hand, if one molecule has a hydrogen atom bonded to an element that is very different from the other molecule's hydrogen, they are less likely to swap.
So, imagine that two people are sharing toys. If they both want the same toy, there might be an argument over who gets to play with it. But if one person has a toy that the other person doesn't really want, they might be more willing to trade. It's kind of the same with atoms sharing hydrogen atoms - if two molecules have similar "toys" (in this case, the element that the hydrogen is bonded to), they might be more likely to make a trade.
Hopefully that helps you understand Grimm's Hydride Displacement Law a little better!