The flag of the Netherlands is a horizontal tricolor of red, white, and blue. Introduced in 1572, it is one of the first tricolors and the oldest tricolor still in use today. Since 1937, the flag has officially been the national flag of the Netherlands and of the Kingdom of the Netherlands. (source)
According to Ancient and Heraldic traditions much symbolism is associated with colors. The colors on the Dutch flag represent the following: white - peace and honesty; red - hardiness, bravery, strength & valor; blue - vigilance, truth and loyalty, perseverance & justice. (source)
Thank you Agnieszka!