The specific rules agents and publishers set for how they want to receive your query, manuscript pages, and other materials.
Submission guidelines are the instructions an agent or publisher provides explaining exactly what they want you to send, how they want you to send it, and any formatting requirements. These typically cover things like: query letter length, whether to include sample pages (and how many), synopsis requirements, file format preferences, email subject line formatting, and whether the agent is even open to queries at all. Every agent has slightly different preferences, and these guidelines are usually posted on their agency website or QueryTracker profile.
Following submission guidelines is the easiest way to show an agent you're professional and respectful of their time. It's also the easiest way to get rejected if you don't. Agents receive hundreds of queries a week. When someone ignores the guidelines, it signals either carelessness or a belief that the rules don't apply to them. Neither is a good look. Reading and following the guidelines costs you ten minutes and dramatically increases the chance your query actually gets read.
Send exactly what is requested. No more, no less. If they want the first ten pages, don't send fifty because you think your book 'really gets going' in chapter three.
Some agents want everything pasted into the email body. Others want attachments. Some want specific file formats. Check every single time. What worked for one agent may get auto-deleted by another.
Agents open and close their query inboxes regularly. Always check their current status before sending. Querying a closed agent wastes your time and may blacklist you from querying them later.
Most guidelines ask you to personalize your query. Even when they don't, a quick sentence explaining why you're querying this specific agent shows you've done your research and aren't just carpet-bombing every agent on QueryTracker.
Pick three agents from your query list and pull up their submission guidelines side by side. Create a comparison chart noting: what materials they want, format preferences, any specific dos and don'ts, and whether they're currently open. Notice how different each set of requirements is. This exercise will save you from careless mistakes when you start querying for real.