This is really a question about the format of conventions and events. You don't give much information about your company or the event, but many kinds of conferences and conventions are divided up into main events which almost all attendees will watch (often called keynote, or as here main stage) where someone will deliver a lengthy individual speech, and panel events which may be smaller and of more specialist interest.
Panels have multiple speakers, usually each speaking for a short time (this is the panel speech), then more time devoted to audience questions or discussion. It's usually the case that the keynote speech will be of substantial length (e.g. 1 hour), and possibly be more spectacular (with slides, AV technology, video, lighting, etc), while a panel may have multiple short speeches from the different people on the panel (unless it's purely a question and answer format): speech length will depend on the number of panelists and precise format, but there may not be Powerpoint or other presentational aids, unlike in a keynote. As well as charging by length of presentation, speakers may also charge by audience or room size, and (in a technical setting) whether they're expected to provide slides etc.
The Wikipedia pages on Academic conference, Panel discussion, and other types of conference go into detail on the typical formats (I'm also basing it on my experience of academic and industry events). But the details will depend on the type of event and the industry/academic discipline, or if it's something else like a political event, fan convention, or charity/third sector event. There are various online guides on how to run a panel discussion and they tend to emphasise that panel speeches should be short with more time given to discussions. Individual speakers bureaux (e.g. JLA) and agents will doubtless have specific definitions about what they'll offer for different fees.