Vihar in Jainism means the movement or wandering of ascetics (munis and deras) as they travel from place to place, mainly to seek alms and to study or teach. It is a disciplined practice rooted in ahimsa (non-violence) and careful conduct.

Key points:

• Night travel (rātri-vihāra) is generally avoided for monks and nuns because it increases the risk of harming small living beings and disrupts discipline. In rare emergencies it may be allowed under strict guidance. ( jainknowledge.com)
• Monastics typically wander during the year, but during the monsoon retreat (chaturmasya) they stay in one place, reflecting restraint and focus. This covers the seasonal rhythm of vihar rather than constant roaming. ( jainknowledge.com)

If you want a concise reference on the practice and its practical rules, you can read more here and here .