For detailed rules on Encumbrance see Hero System 6e VII page 45 thru 46. I have condensed the rules here.
Why worry about encumbrance?
In the Science Fiction genre it is very rare to see characters using body armor except in the most dire of circumstances (i.e.. see Star Trek, Star Wars, Babylon 5, and Firefly). And even warrior societies, such as the Klingons, would wear limited armor, which doesn't seem to have done much to prevent serious wounds from most weapons.
By including encumbrance in the game this will minimize the desire to find heavy armor and also insist on wearing it everywhere, even if it is inappropriate. Characters will need to pick their fights carefully. They will also need to value the concept of retreating and regrouping.
The rules are very simple. You need to calculate how much weight your character is carrying around. You should calculate it based on 'every day' attire and 'battle' attire. Consider weapons, equipment, armor, and any other items you may carry. If the items are part of the equipment list I have provided then the weight is there. You should then add an additional 2 pounds of weight to account for clothing, footwear and any items in your pockets.
Then you consult the following two tables.
On the first table you find your characters strength in the left column. You then find the column where the number is less than or equal to the weight you are carrying and refer to first row and the associated column. This will give you a % of your carrying capacity.
For instance if your character has a 10 Strength and is carrying 43 pounds of gear then you are carrying 24% of your carrying capacity
The next table provides you with the information you need to know how the equipment you are carrying will impact your character's abilities. This is based on the % of your carrying capacity. Using the previous example the character would be encumbered and suffer a -1 DCV and -1 to all DEX based rolls. At higher levels of encumbrance, the character becomes even more impaired.