The condition in which you find various pieces of equipment ranges greatly. This is the apocalypse after all, and you will need to use the Engineering skill to figure out what you need to repair an item. Items you craft are not always in perfect condition and often require more maintenance before they work well.
This can be done by holding your left mouse button over the item, selecting the cheery-looking face of Vaultboy and then selecting the engineering skill. If you already have the items needed in your inventory, it will automatically be repaired and a message will appear along with the associated skill you increased (if you increased a skill).
The condition of weapons greatly affects it's reliability, which left unchecked can lead to Misfires and Jams. Some weapons, such as the AK series (AK-47, AKs-74, RPK, etc) have the reliability trait which makes jams uncommon and misfires rare.