Imagine you are trying to have a private conversation with someone who is standing two feet away from you in a silent room. You can whisper to reach other and you will both have no problem hearing each other. Now imagine you are standing further away. Much further away. You may need to yell now just to hear each other, if you are far enough away. Now imagine again, that you are back to being two feet away from each other, but this time instead of being in a quiet room, you are in a room full of the sound of jackhammers revving chainsaws horrible static sounds and people screaming, alarms blaring, any and every loud noise you can imagine all at once. Good luck trying to hear each other even if you scream back and forth even if you're standing two feet away, let alone further. There is too much interference from the loud noises covering up the actual stuff you are trying to say/hear.

Well, what a jammer does at a basic level, is essentialially create those loud noises to cover up the actual signals sent by the two transceivers trying to communicate, so that they can not "hear" each other.

Something as simple as a leaky microwave oven (as in it does not contain the microwaves within it and they leak out) can cause interference and act as a jammer to 2.4GHz wifi, as a microwave oven uses microwaves that are the same frequency of 2.4GHz. a wired connection works much differently and would be unaffected.