Reiki has a number of ideals or principles.
You may find these in slightly different versions, dependant upon the Reiki healer or teacher you chose. However, they will all essentially mean the same thing. The use of the ‘just for today’ part is so that the ideals are manageable! We couldn’t commit to never getting angry or never worrying ever again, but we can make the effort to not do so in the here and now.
Whether you decide to become attuned to Reiki or not, these are wise principles to incorporate into your life. It is useful to meditate upon what these principles mean to you and a useful exercise would be to deal with each one separately.
You can also repeat these principles as a mantra every day so that they become part of your subconscious.