"Use it or lose it" is one of those time-worn phrases that has many meanings.
From my perspective, I think there is more to it.
This phrase often gets used in a physical fitness sense. If you don't exercise regularly, you will lose the gains you've made. I think it goes a step beyond this.
If you don't exercise, you will always have a level of base fitness. Most people seem to live at this level. If that level had a label, it would read "bare minimum".
Consider this in a business context. If you are not working out, not using your talents and the tools available to communicate those talents to the market, you are putting in the "bare minimum" amount of effort to stay in business. I don't think any of us intentionally do this, but it's what will inevitably happen when you fail to apply focus to your activities.
It is in your best interest to always be working out in your business. That means putting in more than the bare minimum amount of effort in all areas of your business. Whether it's marketing or bookkeeping or keeping up with technical developments, nobody can afford to rest on their past accomplishments or existing knowledge. This is not an era that rewards inaction.
You can choose to start working out today. In the same way that you would work out physically, work up to a higher level of activity you can handle. Once you're comfortable there, keep working and improving. Soon your business will be more fit and you'll be sleeping better at night. Enjoy the day!