The most important thing in music is rhythm, the most important thing to rhythm is the beat, and the most important thing to the beat is its steadiness.
I can play a familiar melody such as “Mary Had a Little Lamb,” and make it completely unrecognizable by radically changing the rhythm. However, it is still easily recognizable if it is played in a serial tone row, (maintaining the melodic shape while using large leaps and wild chromaticism) but keep the original rhythmic patterns.
There are two components to a secure sense of rhythm. First you must KNOW where in the score the beat falls. The beat can be any note value assigned the primary rhythmic motive function. You must understand where in the score these beats occur.
The next part is the “Loving”. You must have a physical sense of the beat. There is no guessing allowed in the beat. Clap your hands, stomp your feet, jump up and down, tap a foot, tap a toe, count out loud; do something to physically feel the presence of the beat or pulse.
Now put these together. Know where you belong in the score as these beats you feel come by. No matter what, you must be where you belong! If your playing is controlled by a steady, known beat, with a thorough understanding as to where you belong in the score with the beat, you will have a secure rhythm. Failure will make your playing rhythmically unintelligible.