A "default" is what the computer is programmed to assume.
For example, in a word processing program, it may assume you want 1 inch (or 2.5 cm) margins. If you don't tell the computer otherwise, it uses the "default" margins, for every new document.
Or in the Microsoft Solitaire program, it may assume you want it to draw 3 cards, every time you click on the deck (it can be changed to draw 1 card, each time you click on the deck). This setting is a default.
The defaults include any settings in a program, that are already set when you first start using a program.
Defaults make it much easier to use a new program, when you first get it. Otherwise, you would have to learn how to adjust and set all the settings, before you could type a single page in a word processor. (The first word processor that I had, back in the mid 1980's worked this way - it had NO defaults. Defaults are definitely a great idea!).
Defaults also save time, because most of the time, most users use the same standard settings all the time. This way you don't have to tell the computer every time that you want the same margins you had last time.
You can also "change" many of the defaults in most programs, so every time you start that program, it will use the settings you chose, instead of the defaults that the program came with.
For example, in Microsoft Solitaire, its default is to draw 3 cards every time you click on the deck. You can change it to draw 1 card, if you want (to do this, click on the Game menu, and click on Options, to find this setting). If you change Microsoft Solitaire to draw only 1 card, you will have changed the "default" so that every time you play in the future, it will only draw 1 card (unless you change it back again).
In a word processing program, however, if you change the margins, the new margins will only be set for that one document. When you start another new document, it will use the (old) default margins again, unless you change the programs "default" margins. So, you can change the margins (and many other settings) for the one document you are working on, OR for all new documents you make in the future (this is changing the defaults), Because there are two possibilities, it can be trickier to change the defaults of some programs.
Normally a word processing program will assume you are only changing the settings for the one document you are working on, so it is harder to change the defaults. Different programs work differently here. If you are in the "Page Setup" dialog box (window), it may have a button that says "Default" on it, or a program may have another way of doing it (that you would find out about by looking in the Help file, under the Help menu, for that program only).
The most important thing is to know about defaults, and that you can change your settings for one new document, or you can change the defaults (the settings for all your new documents). Then you can look for how to do it, for the program you are using.