Inference is using a trained model: you send input, the model computes an output. Training happens once (or rarely) at great cost; inference happens on every single request, which is why its speed and price dominate the economics of AI products.
When news mentions cheaper or faster models, it usually means inference got more efficient — the same capability at lower cost per answer, which is what makes new applications viable.
Every message you send to a chatbot is an inference call: the trained model reads your tokens and generates the reply token by token.