1 Timothy 4:8 (NKJV)
When you grow up in Wisconsin, like I did, you have some clothes that you will only use in the Wintertime and some clothes you will only use in the Summertime. For example, in January, a Winter coat is essential, but you don't need to wear it in August. Or, if you have a pair of sandals, you could wear them in July, but you would not want to wear them in December. So, there are some things that are only useful at some times, but something like shoes are useful items of clothing for every season.
Just like there are some types of clothing that are only beneficial at some times during the year, and some types of clothing that are useful year-round, there are things in life that are like that too. As Paul pointed out in 1 Timothy 4:8, bodily exercise is good, but it is only useful in this life, while godliness is useful in this life and in the life to come. There are a lot of things we can do in our life that have some benefit, but a life lived for God is something that has present and future benefits.
The word godliness in 1 Timothy 4:8 means, piety, towards God, and reverence. What this is telling us is that living a life that honors God, a life that puts godly character in first place, a life that is based on God's Word, is something valuable today and in the future. While things like exercising our body are good and helpful, doing things that honor God and are based on His Word are even better! By living a godly life today, we are putting ourselves in the place where God can work in our life, and we are laying a good foundation for the world to come.