A lahar can be explained as a volcanic water-based debris flow. If that seems a little complicated, breaking it down will help: it is extremely fluid and mostly water (as opposed to other types of debris flows which are clast-based and mostly debris).
Dangers?
Lahars are extremely devastating to the environment and to communities in their paths. As far as structural damage goes, buildings and bridges are frequently leveled due to the impact of debris. Bridges may catch the debris passing under like resulting in the destruction of the bridge and complications in the flooding. Frequently, entire communities may be buried in mud and debris so that seeking shelter in-doors is futile. Comparisons may be drawn to instances where a man-made dam has broken and downstream communities have been impacted. Lahars are extremely important to volcanologists and the people who live near active volcanoes because they are common and can be very deadly.
What can people do to avoid danger from this movements?
Volcano Disaster Assistance Program, which is designed to be a mobile disaster response team capable . One of the primary hazards volcanologists now look for are lahars, which can strike with devastating force at distances far from the volcano. While it remains difficult to estimate the size lahars, it is very possible to forecast their existence and paths they may follow.