Runes have different properties based on whether they are inserted into armor, shields, or weapons. However, the reverse isn't true you can't lower an item's cLvl by inserting a lower-level Rune into a socket. Socketing a cLvl 21 rune into a cLvl15 item will increase the item's level to cLvl 21 to match the rune. If your character is below the cLevel of that rune, that character cannot use that rune, nor use any gear item where this rune is in place. Runes have a minimum character level (cLevel). Runewords have their own sets of rules, and thus are handled in a separate guide: You can sequence some Runes into powerful Runewords, which create more properties than just the runes alone. There are four different types of gear that have sockets:

Runes are small inscribed stones that can be used for specific purposes: Runes in Diablo II: Resurrected Patch 2.7 & Ladder Season 4 Runewords in Diablo II: Resurrected Top 10 Leveling Runewords There are separate guides about Runewords, which you can visit here: There is a full table with all known Runes in the game and their properties. This guide covers the basics about Runes, including how to use, remove, and upgrade them. They are not usable for Classic players who choose not to use the LoD expansion. Runes are only available for players who are using the full Expansion mode of D2:R, which includes the Lord of Destruction (LoD) expansion. Combining Runes together into specific Runewords changes a gear piece's quality to a Runic Item. Each Rune has a name and properties, and can either be added singly to an item or combined into powerful Runewords. Welcome to Wowhead's guide about Runes, magical items that can be inserted into socketed armor and weapons found in Diablo II: Resurrected (D2:R) Patch 2.7 & Ladder Season 4.

Diablo 2 resurrected runes list