The grease pencil is Blender's 2D drawing system. Initially, it's purpose was to allow artists to sketch scenes out onscreen, in much the same way that CGI artists a few decades ago used literal grease pencils  to draw on their glass computer monitors.
Nowadays, the grease pencil is a full 2D art system, allowing for hand-drawn 2D art to coexist with 3D assets or tracked with camera footage. The Blender Foundation made a short film called "Hero"  to show it off.
> How do I select all (UV) verticies?
On the bottom edge of the UV editor window is a menu bar. From the left, the menus are editor type (shown as an icon), "View", "Select", "Image", and "UVs". Farther to the right in that bar is the widget that lets you set whether you are selecting verticies, edges, faces, or islands. In the "select" menu is "(De)select All", and a label indicating that the keyboard shortcut is "A". (This shortcut is universal to all of Blender's editors (except the text editor, where it is Ctrl+A).
If you have anything at all selected, then "(De)select All" will deselect everything. If you have nothing selected, it will select everything.
> How do I select multiple via box?
In Blender, this is called "border select". In the same "select" menu I described above, it's the bottom option (the one nearest your cursor when you open the menu). It also has a universal shortcut, indicated in the menu: "B".
> How do I do numeric input?
There are two ways I can interpret this. One is "given a number box, how to I change the value"? There are three ways that you can change the content of number boxes in Blender.
1. Click on the arrows on the left or right sides to decrement or increment the value.
2. Click in the middle, type a value or math expression, and then either click outside the box or type "enter".
3. Click and drag the box, from the middle, to the left or right.
The other way I can interpret the question is "how can I specify the coordinates of a UV vertex"?
Once you have one or more verticies selected in the UV editor, you should see some properties and editing widgets in a tall rectangular area on the right edge of the UV editor window. If you don't see it, you can reveal it by either clicking on the "+" button near the top of that edge, by going to the "view" menu and selecting "properties", or by hitting the "N" key while your cursor is over the UV editor.
In the top of the UV properties shelf I just described is a section called "UV Vertex". (It's called that even when you are in edge, face, or island selecting mode.) It has two numeric boxes in it labeled "X" and "Y". These have the coordinates of the most recently selected vertex. Changing the contents of those boxes moves the selected vertex to those coordinates, and moves all other selected verticies by the same amount from their previous locations.
An essential Blender video to watch is Captain Disillusion's presentation at Blender Conf: https://www.youtube.com/watch?v=1qSTcxt2t74