An informal meeting is any meeting that isn't highly structured and doesn't have a specific length, scope or other rigid component such as minutes. Informal meetings are often impromptu and focus on small problems.
The main advantage that an informal meeting has over a traditional formal meeting is flexibility. If a small problem has arisen on a business project, it is often more efficient to address the issue right away with an informal meeting where only the affected personnel are involved. This avoids the trouble of scheduling a meeting where all team members are available. An informal meeting may also be preferred when there is no specific topic in mind, but brainstorming or discussion of broad matters is needed.