A melodic phrase is a single line of melody. Originally a phrase was sung in a single breath by the singer, with a rest between phrases for the singer to take the next breath. Some modern-day songs use longer phrases that require more than one breath, but it helps to remember that phrasing is at least symbolically linked to breathing. A phrase is a breath of music.
One of the most effective ways to give shape and pacing to the song as a whole is to vary the length of melodic phrases. For example, you might have 4-bar phrases in the verse and 2-bar phrases in the chorus. The different phrase length gives the chorus an unmistakably different feel from the verse.
Another common formula is to use shortened phrases at the end of the verse, to create a sense of increased momentum heading into the chorus.
Many ordinary songs use 4- and 8-bar phrases all the way through. It's unfortunately so commonplace that using any other phrase length anywhere in your song will help you stand out as interesting and different. Look to the Beatles catalog for some great examples of a flexible approach to phrase length.