A way to give each sequence item its position.
Without positional encoding, a sentence is fridge magnets in a junk drawer. All the words are there, but 'dog bites man' can turn into 'man bites dog.'
It gives each word a seat number. Then models keep order in long writing and code.
Transformer
Positional Encoding lets a Transformer know each word's place.
Self-Attention
Self-Attention can compare words, but it needs help with order.
Embedding
Positional Encoding is often added to the word Embedding.