Glossary
Token
Short answer
The smallest unit a model breaks a prompt into; it defines the prompt length limit.
Models read a prompt token by token, not word by word. A token usually corresponds to a word or part of one.
Every model has a token limit, and the end of a prompt that exceeds it is usually silently ignored. That is the technical explanation for why very long prompts do not work.
The practical consequence: put the most important information at the start of the prompt. Details left at the end are more likely to be disregarded.