There is a common demonstration, that we'll do as a thought experiment, when talking about priorities. It has a person fill a bucket with sand, then try to add as many pebbles to the bucket as possible, following that they are to add larger gravel, then fist sized rocks. The obvious thing that happens is the bucket fills with sand and they are not able to add much else to it. Then they are instructed to add them in the reverse order so that larger rocks first, then gravel, then small pebbles, then the sand. Doing it this way more large items are able to be added to the bucket then the smaller ones fill in the space left by the larger items.

This is used to demonstrate the necessity to prioritize the important items and fit them in first then go in order down to the least important. The larger rocks represent higher priority item whereas the smallest grains of sand represent the least important items. If you fill your time with low priority items you won't have space for the higher priority ones, but if you start with the larger ones then you'll be able to fit smaller and smaller ones into the gaps.

Where it gets tricky is in determining where an items falls in priority. Typically the very high and the very low priority items are obvious. A medical emergency is a very high priority item, responding to a friend request from someone you haven't met is a very low priority item. The rest of the items may not be as obvious and be more difficult to prioritize.

Prioritization can help you to maximize your efforts onto the items and tasks that will give you the most reward. However, a problem with the bucket example (demonstration) is that once you fill in all the gaps with the smallest grains of sand there is no room to breathe in the bucket. It becomes difficult to make adjustments when everything is packed so tight into the bucket. The same is true of your schedule, when you pack too much into it you don't have capacity to make adjustments. This can work for a while, especially when you are at crunch time but shouldn't be the norm as it is not maintainable. Use the information discussed here to guide you when you are planning out your priorities and don't forget to leave some room to breathe in your schedule.