f stands for float, verses d stands for double etc.
There are four stacks of matrices: projection, modelview, texture and color.
Before you call glLoadIdentity to set the values of a matrix, you need to specify which matrix stack you want to set. So you call glMatrixMode() to set the “current” matrix.
In your 2 lines of code, glLoadIdentity() will set the value of projection matrix as identity matrix.

glVertex*() defines the position of a vertex which can either be rendered as a single point or can be a vertex in a sequence of vertices making up a more complex entity, like a line, a triangle, a quad or a more general polygon. The name of the function is a little misleading as it really defines the position in object-space, which is only one of potentially many so called vertex attributes a vertex may be associated with. Other vertex attributes may include a normal, a vertex color, a texture coordinate and so forth.

Note: glVertex*() is legacy OpenGL! Modern OpenGL uses different ways to specify geometry and vertex attributes. If you’re just now learning OpenGL, please learn modern OpenGL, i.e. OpenGL 3.0 or higher.