There are a lot of common Dungeons and Dragons (D&D) terms in Baldur's Gate 3. This makes sense as the developers Larian Studios follow the D&D 5.0 manual to create the various rules and guidelines for how combat, testing of skills, characters and general gameplay work. Baldur's Gate, originally released in 1998, used D&D 2nd Edition rules, and Baldur's Gate 3 continues to follow that tradition with the revised version. You can expect to see various terms not previously used in Larian Studios games, such as focus, rough terrain, or even 1d4, 1d6, 1d8, and 1d10.
Terms like 1d4 and 1d6 are commonly used whenever you talk about a weapon or spell your character has at their disposal. For example, a shortsword has 1d6 slashing damage, but there is no detail for what that means when you hover over the term.
Anytime you see something that uses 1d per something, it is dice. The term "1d" means it uses a dice, so when your character successfully hits another character in battle, behind the scenes, he rolls a dice of the next number. So when your character launches an attack on an enemy, they roll a six-sided die to see how much damage they do. This means that you can expect an attack to damage an enemy for one to six points of damage. The end result is usually higher because your character also adds their skill score to their damage die, which you can see on their gear page at the bottom.
Almost all of the attacks or spells your characters use in Baldur's Gate 3 have one of these dice rolls attached to their description. This gives you a better idea of how powerful these attacks are, and for those who like to use spellcasters, you can expect to see some that have 2d4 or 3d4, meaning the attack rolls two four-sided dice. or three to four sides. dice, respectively. So if you have the option of using an attack that uses 1d10 or one that uses 1d4, you will probably want to use 1d10 because you can potentially roll a four or more. However, there is a lot of luck in dice rolls, and there is always a chance that you can roll one.
Be sure to recheck the spell or attack before using it to see how much damage it does to an enemy. Depending on how many dice your character rolls, you might be better off using a different choice.