Subagents refers to the idea that rather than thinking of the mind as an entity with one set of goals and beliefs, it includes many independently acting components, each of which might have varying goals and beliefs. One intuitive way of expressing this is the expression "one part of me wants X, but another part of me wants Y instead".
While the name implies some degree of independent agency on part of the subagents, they may also be viewed as being more passive entities. For example, the "parts" in the above example may be considered different sets of beliefs, accessed one at a time by the same system.
The Multiagent Models of Mind sequence explores the notion of subagents in detail. Akrasia (acting against one's better judgment, such as by procrastinating) may involve subagent disagreement. Internal Double Crux is a specific technique for getting subagents to agree with each other.