A sidehop is an essential manouvre in bicycle trials that is used to jump from the ground to a higher object. It is similar to bunnyhopping except that you're not moving when it is performed.
To do a sidehop you'll first need to learn to trackstand, i.e. stand still without putting your foot down (taking a dab). Once you can do this, lock up both wheels with the brakes, lean back to raise the front wheel while pulling it in the desired direction and then "unweight" the back wheel to pull it up. Don't pull up the back end by shifting weight to the front wheel. Instead, lift the back wheel by throwing the bike forwards. If you don't understand this explanation I'd recommend reading my bunnyhopping node first. Also, look around for videos of people performing moves. One good site is www.trials-online.com
There are other varients of the sidehop including the nosepick sidehop, where the front wheel is placed on the object before the back wheel is completely raised. This helps to pull up the back wheel by shifting all your weight onto the front wheel (a complete unweighting). But all these varients use the original sidehop.
When learning how to sidehop I would recommend mastering it on a flat surface before attempting to hop onto things. Once you become more proficient you can use it to climb onto higher objects, up to 4 feet for expert level riders.