They had different customs and religious ideas. Let's see what he wrote, and consider how the principles might apply to situations today. Some Christians are weak in the faith. People grow in faith through love and acceptance, not through condemnation. Christians who think they are strong are sometimes tempted to judge others, to look down on people who have different ideas and customs.
Some matters are disputable.
The beliefs and practices that some Christians think are important, other Christians do not. One side may be right, but Paul tells us to view the matter as debatable. Paul does not tell us why some people avoided meat. Perhaps they were influenced by ascetic false religions, but it seems more likely that the concerns came from Judaism. Some Jews avoided meat because they could not be sure that the animals had been properly killed and bled.