A "Hammer - On" refers to the technique of playing a note without actually picking the string with the right hand.
The sound is produced by striking the string with one of the left hand fingers. The Hammer - On follows another note that has been played by a Hammer - Attack.
The Hammer - On effect is indicated by the curved line, and the small "H" above the Hammered - On note.
In the next exercise, the C note is played as a Hammer - Attack (with the 1st finger of the left hand), and the 2nd finger "Hammer - On" firmly to produce the sound of a D flat note. The Hammered - On note is always higher in pitch than the note preceding it.
Remember that the second note D flat, is not picked; the sound is produced entirely by the 2nd finger "Hammering On" to the string.
The numbers on the staff indicate what finger is used. E.g., 1 means use the first finger.
The fingers of your left and right hand are numbered as such.