You are carrying more stuff than you can hold. In you Pip-Boy under items on the top of the screen there should be something like this:
WG 247(random number)/230(random number). The first number is the amount of pounds you are currntly carrying (each weapon/armor/med (some)/misc item has a weight. Look at the WG by the item to know the weight.
The next number is the amount of weight you can carry, and in this example you are trying to carry more than you actually can, so you have become over-cumbered. Meaning you will be slowed down greatly. To get rid of some items so you are not over-cumbered, go to your Pip-Boy and press square on an item to drop it, therefor losing weight.
Some armor/drugs (like power armor) can increase you strength, making it so you can carry more.
Some ways to reduce your weight without wasting your precious salvage:
Put on the Vault 101 Utility jumpsuit for the +5 Repair and repair worn guns with similar guns in your inventory. Generally, repairing a gun then selling it will gain you more value than selling the individual, more worn out guns. The only time you might not want to do this is if you're trying to accumulate the 20 Chinese Assault Rifles for Pronto in Paradise Falls.
Find a container in which to put things. My favorite place to stash stuff is Tepid Sewers once I clear it out, but there are many other places where you can safely store stuff and you don't have to 'own' it beforehand.
Use a companion as a pack mule. Especially if you give them power armor to wear, they can carry a lot of equipment for you. Just don't forget to pull it off them when you go to a shop.
Build weapons from schematics. The weapons will generally weigh less than the combined weight of the parts, freeing up some more carrying weight, and generally the weapon will be worth more than the combined value of the parts (a higher repair skill increases the value).
Take the Strong Back perk (STR 5, END 5 minimum required).
Only take junk with a weight-to-value ratio of 1:20 - Vodka, Nuka-Colas are a couple examples. Most guns fit this ratio (or better), too. Hording things is okay if you're nearby somebody you can sell to and can stand walking slowly for a little while. Remember that merchants require 73 hours to replenish/change their wares, so you may need to use the wait feature a lot if you are too encumbered to fast travel.