Use the adjective every to talk about all examples of something or all the members of a group. If you invite every classmate to your party, you're asking all 30 of them to come.
When you plan a trip to visit every national park in the US, you are intending to travel to 59 different parks — all of them. When you talk about time, you can also use every to explain how often an event occurs: "I've been waking up every hour lately." Every was originally a contraction of æfre ælc, "each of a group" or "ever each" in Old English.