V1 and V2 are different techniques used in cross-country skiing, specifically in the skate skiing style. V1 is a diagonal technique, where the skier pushes off with one ski while the other ski glides forward. The skier then alternates the pushing leg with each stride. V2 is a double poling technique, where the skier pushes off with both skis simultaneously while also using their poles for propulsion. V2 is considered to be more efficient than V1 and is typically used on flatter or uphill terrain.
Nordic Lab has a great video showing the differences: Nordic Ski Lab – Skate Skiing Techniques Explained