What is it? Edit
Overencumberment is when you are carrying too many items. It slows you down when walking around. When overencumbered, the bottom left corner will be flashing the word "overencumbered" and the player will be slowed down dramatically. You can veiw the amount of items you can carry in your inventory by pressing I or TAB (By default). From there, you can veiw how much you can carry and how much you currently have.
How do you upgrade?Edit
To increase the amount of items you can carry, you can go to a trader. Cick on "Upgrades" then buy "Strength" to upgrade.
Why should you upgrade?Edit
It is very important to upgrade as it is very dangerous to be overencumbered in the middle of a horde. If you are in the middle of a zombie horde with Spitterssurrounding you, you'd be in a very gooey situation. Overencumberment can come at any moment given that you are carrring more than you can handle.