Subpanels are a list of related records. If a record can only be associated with one record, you'll see the relationship represented by a field. For instance, a site can only have one Primary Contact so when you look at an Account record, you'll see a field labeled "Primary Contact".
But if multiple records can be associated with a single record, you'll instead see those related records in a subpanel. For example, a Site can have many coaching details so if you open an account record and scroll down, you'll see a subpanel labeled "Coaching Details" that will list all of that site's coaching details.
BUT WHY SHOULD I CARE ABOUT SUBPANELS!?!?
The reason you should care about subpanels is that they're an easy way to quickly see what records are associated with the records you are looking at. If you're looking at a requisition and want to see all of the transfers associated with that requisition, you can just scroll down and see all the transfers in the "Transfers To" and "Transfers From" subpanels. Without subpanels you'd have to go the transfers module, create a filter where the "from requisition" is the requisition you were looking at and then you'd need to replace that filter with a new one where "to requisition" is the requisition you were looking at.
This article will walk you through how to arrange Subpanel layouts by order of most important to your daily work. Once you edit One Record in a Module that layout will be saved for all records in any Module.
For the purposes of this article I will be walking you through how to change the Account Subpanel Layout. You can edit these Subpanels in almost all of the Modules in Sugar that have multiple Subpanels!
1. Click on an Account Record . 
2. Scroll down to the Subpanels.
3. Click, hold, and drag these individual Subpanels up or down based on those that you use more and need more immediate access to when visiting an account record. Once you have moved the Subpanel Sugar will automatically save your changes!