Minecraft supports adding Custom Model Data to various blocks and items. It would look strange if these custom blocks had the same descriptions as normal blocks, so Item Descriptions supports custom descriptions for these items. The format differs depending on your version.
1.21.4 and above
As 1.21.4 makes custom model data more complicated, Item Descriptions will pull from the string and float arrays. If you give yourself a Mace with a Custom Model Data component with the first element of the string array set as "bat", you would add the following entry to your resource pack's language file (en_us.json for English U.S.).
For more information, Ewan Howell has a tutorial on custom model data alongside a resource pack.
/give @s minecraft:mace[minecraft:custom_model_data={strings:["bat"]}]
"lore.minecraft.mace.custommodeldata.bat": "This is no Mace at all, it's a baseball bat!",For other items, you would use a similar description.
"lore.<item_namespace>.<item_id>.custommodeldata.<value_of_custom_model_data_first_string>": "<Description here!>",1.21.3 and below
Assuming you've set the value of a Carved Pumpkin's Custom Model Data to 121234567, you would add the following entry to your resource pack's language file (en_us.json for English U.S.).
For more information, Geegaz has a tutorial on Custom Model Data alongside a resource pack.
"lore.minecraft.carved_pumpkin.custommodeldata.1234567": "Hello!",For other items, you would use a similar description.
"lore.<item_namespace>.<item_id>.custommodeldata.<value_of_custom_model_data>": "<Description here!>",