The [שולחן ערוך [אורח חיים צח:א says המתפלל צריך שיכוין בלבו פירוש המילים. The משנה ברורה, citing the מגן אברהם, says ואל יכוין האדם בשמות ויחודים, רק יתפלל כפשוטו. Nonetheless, "כפשוטו" can include some depth. These pages are some of my thoughts about these words, along with some of my other thoughts that fit in the themes presented. I can't claim that I concentrate on all of these ideas while davening (or even that I concentrate at all), but at least these are the things that I think I should think about.
These pages were inspired after I had learned, in rapid succession, Rav Hirsch's Horeb, Rabbi Schwab's Rav Schwab on Prayer, and Rabbi Kleinman's Praying with Fire. All of these go through the davening phrase-by-phrase, explaining the meaning and intention of each ברכה, emphasing more than the translation—it is the כוונה, the underlying intention, that we should have in mind.
Everything here is my own opinion. I am not a rabbi or תלמיד חכם and do not claim to be one. I try to cite sources when I can, but I may have missed something or misinterpreted something so badly as to make it nonsense. For that I apologize. Feedback is always welcome; please email me with your thoughts and comments.