A codec, introducted in Java Edition 1.16, is a specification of conversion between any type of object (such as LootTable, Advancement or BlockPos) and any type serialized from (nbt, json, etc.). A codec is a combination of encoder and decoder. An encoder encodes an object to serialized form, and a decoder decodes the serialized from into objects.

For example, loot tables are written in json forms in data packs, and a loaded as LootTable objects in the server. To load loot tables from the jsons in the data pack, the codec is used. There are many pre-written codecs in Minecraft, each of which, is used for a specific type of object, but can serialize or deserialize between it and different types of serialized from. For example, LootTable.CODEC can convert LootTable objects into jsons and nbts, and can also convert jsons or nbts into LootTable objects.

Codec was introduced in 1.16, but has been in increasingly widely used in Minecraft since 1.20. For example, LootTable, Advancements and Text, previously serialized or deserialized in other manners in older versions, but now using Codec. Item components, introducted in 1.20.5, also use codecs to serialize.

When you serialize or deserialize, you need to specify a DynamicOps, which specifies each concrete action in the serialization or deserialization. Mose common used are JsonOps.INSTANCE and NbtOps.INSTANCE. The following code takes an example of converting between BlockPos and NbtElement.

    // serializing a BlockPos
    final BlockPos blockPos = new BlockPos(1, 2, 3);
    final DataResult<NbtElement> encodeResult = BlockPos.CODEC.encodeStart(NbtOps.INSTANCE, blockPos);
 
    // deserializing a BlockPos
    final NbtList nbtList = new NbtList();
    nbtList.add(NbtInt.of(1));
    nbtList.add(NbtInt.of(2));
    nbtList.add(NbtInt.of(3));
    final DataResult<Pair<BlockPos, NbtElement>> decodeResult = BlockPos.CODEC.decode(NbtOps.INSTANCE, nbtList);

As seen in the code, the result is not directly NbtElement or BlockPos, but a DataResult wrapping them. That's because errors are common in serialization, and instead of exceptions, errors in the data results often happens. You can fetch the result with result() (which may be empty when error happens), or fecth the error message with error() (which may be empty when error does not happen). You can also directly fetch the result with getOrThrow() (or Util.getResult in older versions).

    // get the result when succeed
    final NbtList nbtList = new NbtList();
    nbtList.add(NbtInt.of(1));
    nbtList.add(NbtInt.of(2));
    nbtList.add(NbtInt.of(3));
    final DataResult<Pair<BlockPos, NbtElement>> result1 = BlockPos.CODEC.decode(NbtOps.INSTANCE, nbtList);
    System.out.println(result1.getOrThrow().getFirst());
 
    // get the result when error
    final NbtString nbtString = NbtString.of("can't decode me");
    final DataResult<Pair<BlockPos, NbtElement>> result2 = BlockPos.CODEC.decode(NbtOps.INSTANCE, nbtString);
    System.out.println(result2.error().get().message());