The slide is a technique which involves a finger moving between two notes on the same string. The finger maintains pressure on the string, so that a continuous sound is produced until the desired note is reached.
The left hand moves from one note to the next, upwards or downwards on the fretboard. The slide is indicated by a line joining two notes. Only the first note is played by the right hand, the second one is entirely produced by the left hand finger sliding up or down the fretboard. The length of the slide can be one fret or as many frets as you wish.
Practice sliding your finger up and down the fretboard. Pick only the first note. The slide is indicated on the music staff by a line leading up to the note you are sliding to. In tablature notation the slide is indicated by a line with the letter "s" above it.
This example demonstrates some typical uses of slides.
To sum things up, here are some pieces which use everything you have learned. These pieces make a lot of use of the open strings, which is one of the features of acoustic guitar playing. You will also find notes higher up the fretboard than you have previously learned. These can be found by following the TAB.
In this final solo the right thumb plays open string bass notes on each beat (called "constant bass style") while the fingers play the melody and harmony notes. The keys of A and E work particularly well for this technique. This piece also uses note bending, particularly in bar 5. This is achieved by pushing the strings slightly towards the middle of the fretboard. To learn more about this, and Blues playing in general, see Progressive Blues Guitar .