On the guitar, "legato" is a term often used to describe a group of techniques that can be useful in playing with a smoother sound. Hammer-ons and pull offs come into this category.
Hammer-ons –
To execute a hammer on you will have to take one of the fingers on your left hand (if you're using a right handed guitar that is) and quite literally "hammer on" to the fretboard at the desired fret / string. Be sure to use enough force to make the note sound but do not strain yourself. You should not be using your right hand for anything at this stage. This kind of hammer-on is called a hammer-on from nowhere and is not used that much in comparison to the next example.
The most common type of hammer-on played is where you play a note (for the purpose of this example the 5th fret of the d string) in the regular way but instead of picking the next note you would hammer on to the next note. Try playing the 5th fret and while the note is still sustaining hammer-on to the 7th fret with one of your free fingers. If done smoothly and with enough force the note that has been executed with a hammer-on will ring out and sound quite smooth as a result of not being picked.
This technique can be used on any string and at any fret and will work no matter what distance is between the two notes. It is good practice to become comfortable with all of your fingers while using this technique as it is quite common and will be an invaluable asset to you later on.
HAMMER-ONS AND PULL OFFS
Pull offs –
Pull offs are the reverse of hammer-ons in a lot of ways, although the name can be a bit misleading.
When performing a pull off although you do pull off the string there is more to the technique than just lifting your finger off of the string. When performing a pull off you must make sure that you catch the string and "pull off" it in either a downwards or an upwards motion. It's important to do this as catching the string and then releasing it will cause the string to sound again without you having to pick the string with your right hand. If you do not catch the string in either a downwards (towards the floor) motion or an upwards (towards the ceiling) motion then the next note will not sound and your efforts will have been in vain.
You can practice this by fretting the 2nd fret of the first string and picking the string to sound it. Once the string is sounded you should immediately "pull off" to the open 1st string. If you have done this successfully then the open e string will have sounded clearly and strongly. Once you have mastered this you can combine the pull off with another hammer-on and play the 2nd fret again by hammering onto it from the open e.
This may all feel quite alien at first as you probably will not have used a lot of the muscles in your hand / arm in this way ever before and it will take a bit of time for it to feel natural but with practice you will develop the muscle memory required to do this effortlessly.