essentially, mget(x, y) takes a tile's x and y position on the map, and returns the tile id of it (a number) . It's often used for collision in platformers.
Fget(tile_id, flag) works in a similar manner, however fget() returns a boolean value. it will return true if the tile ur checking has that flag, and false if it doesnt. To define a tile's id, go to the sprite editor, and click the little switch underneath the sprite editor logo (at the top) You'll then be in advanced mode, and see some new stuff appear. Focus on the numbers with boxes on the left. These are the flags you can use to define how a tile may act. For example, clicking the button next to 0 will make a red dot appear, and will make this tile have a flag of 0. This could be used to determine whether a tile is solid or semi-solid. You can also use multiple flags per tile, but keep in mind you only have 8 of them.
Hope this helps you in some way!
:D