To dissolve or mix into a liquid — to dilute or beat something into a liquid form (e.g., dissolve paint in water, beat eggs).
オムレツを作る前に卵をよく溶いておく。
Beat the eggs well before making the omelet.
絵の具は少し水で溶いてから使ったほうが描きやすい。
It's easier to paint if you dilute the paint a little with water first.
小麦粉を水で溶いて生地を作る。
Mix flour with water to make the batter.