A hammer on is a great way to get from one note to another with speed. It is used in solo techniques and is a great way to really develop strength in those fingers. Watch the video below to find out more about hammering-on. . .
To identify a hammer on within tablature, there would be a little slur joining one note to the other as shown below on the note at the end of bar 1 onto the 1st note of bar 2.