This line from the traditional tune 'Twinkle, Twinkle, Little Star' can be thought of as a single phrase or two phrases (bars 1 and 2, and bars 3 and 4).
A phrase is a single line of music played or sung, which is generally longer than a motif and is felt to form a unit. If any simple melody or folk song is hummed through it will be felt to fall into sections corresponding roughly to the lines of the words, and if any instrumental composition is listened to carefully it will, likewise, be found to fall into sections, these being the music's phrases.
The normal phrase-length is 4 bars (or measures), but 3-bar and 5-bar phrases occur, and there is actually no strict rule as to phrase-length, and the introduction of a change of length often contributes to the variety of a composition. Phrases will often be found to fall into half-phrases, and these into motifs. Two or more phrases will often be found to adhere together making up what is called a sentence.
In the case of simple musical forms. such as hymns, carols, and folk songs, the phrases are often of the same length, usually two or four bars. A consequence of this is that the beginnings and endings of phrases normally compliment each other, for example, if a a phrase in 3/4 time starts on the third beat of bar, it must end with the second so that the next phrase can also begin on the third. Similarly, if the same music is repeated for another verse of the words, the first verse will have to end with the second beat so that the next verse can begin again on the third.
At the end of a phrase, comes a point of rest or relaxation in the music called a cadence, of which there are various types.