I don't have anything against the Galaxy tarot app and use it myself from time to time, but I'm afraid that in order to learn tarot properly, you should invest in a good book or two, and read a lot here on AT. Every good beginner's book will include a version of the Celtic Cross and make you understand the positions. Then you'll also see that every author takes the spread to a slightly different direction.
The book Spiritual Tarot has an extra chapter about basic spreads, among them the Celtic Cross. Joan Bunnings has a book about spreads, but her tarot learning course (recommended!) gives a good explanation of the basic spread here:
http://www.learntarot.com/ccross.htm
I personally interpret the two crossed cards in the center as the two basic impulses at the moment, either a conflict or a synergy, that dominate your life. This is the center of the spread. Above it are your conscious wishes, goals and what you have under control. Beneath are your unconscious motifs and motivations. We all act all the time driven by these two forces, and recognizing them is important.
So this is the "column of consciousness", and it's crossed by the "arrow of time" - past to the left, potential future to the right.
Often, it's enough for me to lay out these cards. I call it the Celtic Wheel and I don't need the additional information given by the four staff cards. But that's a personal choice, I prefer short spreads.
The best explanation of the possibilities of the Celtic Cross spread I read so far is in the book by Marcus Katz (Secrets of the Celtic Cross). In spite of its sensationalist title and in spite of not especially liking the Celtic Cross spread, I read it and found it interesting, a new take on this over-popular spread, and I'll incorporate some of Katz' ideas.