Acts 15:41 (NKJV)
If you have ever done any kind of exercise or physical training, you know that it is something that takes place over a longer period of time and not just on one day. If you are doing a certain exercise, you don't just do it once and see instant results, you keep on doing the exercise to develop the muscles and gain strength. If you are interested in becoming a better swimmer, for example, you don't just spend one day in the pool, you repeatedly train and develop the technique and muscles that are necessary to be a better swimmer.
In the Book of Acts, we read about the Apostle Paul and how he traveled, teaching, preaching, and starting churches. And in his traveling, we can see that Paul went to some places more than once. In Acts 14:22, 15:41, and 18:23, what Paul did is described as "strengthening" the churches and disciples. Different translations may use different words in these verses for the word "strengthening", but the meaning of the word being "to strengthen more" or "to render more firm" is there in every case.
The principle we see from what Paul did is something that can help us today, personally and when dealing with other people; we can further develop and strengthen ourselves and others on a continual basis. We all need a regular time of reading and studying the Bible, time in prayer, and spending time with other like-minded believers. As we continue to do these things, we will develop and add strength to ourselves, and we will be a greater asset to God and to our local church.